[PATCH 0/2] Refactor snapshot vs nocow writers locking

[PATCH 0/2] Refactor snapshot vs nocow writers locking

[Nikolay Borisov]

This patchset first factors out the open code which essentially implements a 
lock that allows to have either multiple reader or multiple writers but not 
both. Then patch 2 just converts the code to using the newly introduced lock. 

The individual patch descriptions contain more information about the technical 
details and invariants that the lock provide. 

I have also CC'ed a copule of the maintainer of linux memory model since my 
patches just factor out the code and I would really like someone proficient 
enough in the usage/semantics of memory barries to review it as well. 

Nikolay Borisov (2):
  btrfs: Implement DRW lock
  btrfs: convert snapshot/nocow exlcusion to drw lock

 fs/btrfs/Makefile      |  2 +-
 fs/btrfs/ctree.h       | 10 ++----
 fs/btrfs/disk-io.c     | 39 ++---------------------
 fs/btrfs/drw_lock.c    | 71 ++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/drw_lock.h    | 23 ++++++++++++++
 fs/btrfs/extent-tree.c | 35 ---------------------
 fs/btrfs/file.c        | 12 +++----
 fs/btrfs/inode.c       |  8 ++---
 fs/btrfs/ioctl.c       | 10 ++----
 9 files changed, 114 insertions(+), 96 deletions(-)
 create mode 100644 fs/btrfs/drw_lock.c
 create mode 100644 fs/btrfs/drw_lock.h

-- 
2.17.1

[PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
to have multiple readers or multiple writers but not multiple readers
and writers holding it concurrently. The code is factored out from
the existing open-coded locking scheme used to exclude pending
snapshots from nocow writers and vice-versa. Current implementation
actually favors Readers (that is snapshot creaters) to writers (nocow
writers of the filesystem).

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/Makefile   |  2 +-
 fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/drw_lock.h | 23 +++++++++++++++
 3 files changed, 95 insertions(+), 1 deletion(-)
 create mode 100644 fs/btrfs/drw_lock.c
 create mode 100644 fs/btrfs/drw_lock.h

diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index ca693dd554e9..dc60127791e6 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
 	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
 	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
-	   uuid-tree.o props.o free-space-tree.o tree-checker.o
+	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
 
 btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
 btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
new file mode 100644
index 000000000000..9681bf7544be
--- /dev/null
+++ b/fs/btrfs/drw_lock.c
@@ -0,0 +1,71 @@
+#include "drw_lock.h"
+#include "ctree.h"
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
+{
+	atomic_set(&lock->readers, 0);
+	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
+	init_waitqueue_head(&lock->pending_readers);
+	init_waitqueue_head(&lock->pending_writers);
+}
+
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_destroy(&lock->writers);
+}
+
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
+{
+	if (atomic_read(&lock->readers))
+		return false;
+
+	percpu_counter_inc(&lock->writers);
+
+	/*
+	 * Ensure writers count is updated before we check for
+	 * pending readers
+	 */
+	smp_mb();
+	if (atomic_read(&lock->readers)) {
+		btrfs_drw_read_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
+{
+	while(true) {
+		if (btrfs_drw_try_write_lock(lock))
+			return;
+		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
+	}
+}
+
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_dec(&lock->writers);
+	cond_wake_up(&lock->pending_readers);
+}
+
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
+{
+	atomic_inc(&lock->readers);
+	smp_mb__after_atomic();
+
+	wait_event(lock->pending_readers,
+		   percpu_counter_sum(&lock->writers) == 0);
+}
+
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
+{
+	/*
+	 * Atomic RMW operations imply full barrier, so woken up writers
+	 * are guaranteed to see the decrement
+	 */
+	if (atomic_dec_and_test(&lock->readers))
+		wake_up(&lock->pending_writers);
+}
+
+
diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
new file mode 100644
index 000000000000..baff59561c06
--- /dev/null
+++ b/fs/btrfs/drw_lock.h
@@ -0,0 +1,23 @@
+#ifndef BTRFS_DRW_LOCK_H
+#define BTRFS_DRW_LOCK_H
+
+#include <linux/atomic.h>
+#include <linux/wait.h>
+#include <linux/percpu_counter.h>
+
+struct btrfs_drw_lock {
+	atomic_t readers;
+	struct percpu_counter writers;
+	wait_queue_head_t pending_writers;
+	wait_queue_head_t pending_readers;
+};
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
+
+#endif
-- 
2.17.1

[PATCH 2/2] btrfs: convert snapshot/nocow exlcusion to drw lock

[Nikolay Borisov]

This patch removes all haphazard code implementing nocow writers
exclusion from pending snapshot creation and switches to using the drw
lock to ensure this invariant still holds. "Readers" are snapshot
creators from create_snapshot and 'writers' are nocow writers from
buffered write path or btrfs_setsize. This locking scheme allows for
multiple snapshots to happen while any nocow writers are blocked, since
writes to page cache in the nocow path will make snapshots inconsistent.

So for performance reasons we'd like to have the ability to run multiple
concurrent snapshots and also favors readers in this case. And in case
there aren't pending snapshots (which will be the majority of the cases)
we rely on the percpu's writers counter to avoid cacheline contention.

The main gain from using the drw is it's now a lot easier to reason about
the guarantees of the locking scheme and whether there is some silent
breakage lurking.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/ctree.h       | 10 +++-------
 fs/btrfs/disk-io.c     | 39 +++------------------------------------
 fs/btrfs/extent-tree.c | 35 -----------------------------------
 fs/btrfs/file.c        | 12 ++++++------
 fs/btrfs/inode.c       |  8 ++++----
 fs/btrfs/ioctl.c       | 10 +++-------
 6 files changed, 19 insertions(+), 95 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index a66ed58058d9..fa8a2e15c77c 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -32,6 +32,7 @@
 #include "extent_io.h"
 #include "extent_map.h"
 #include "async-thread.h"
+#include "drw_lock.h"
 
 struct btrfs_trans_handle;
 struct btrfs_transaction;
@@ -1174,11 +1175,6 @@ static inline struct btrfs_fs_info *btrfs_sb(struct super_block *sb)
 	return sb->s_fs_info;
 }
 
-struct btrfs_subvolume_writers {
-	struct percpu_counter	counter;
-	wait_queue_head_t	wait;
-};
-
 /*
  * The state of btrfs root
  */
@@ -1350,8 +1346,8 @@ struct btrfs_root {
 	 * root_item_lock.
 	 */
 	int dedupe_in_progress;
-	struct btrfs_subvolume_writers *subv_writers;
-	atomic_t will_be_snapshotted;
+	struct btrfs_drw_lock snapshot_lock;
+
 	atomic_t snapshot_force_cow;
 
 	/* For qgroup metadata reserved space */
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 05f215b4d060..ece45e606846 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1125,32 +1125,6 @@ void btrfs_clean_tree_block(struct extent_buffer *buf)
 	}
 }
 
-static struct btrfs_subvolume_writers *btrfs_alloc_subvolume_writers(void)
-{
-	struct btrfs_subvolume_writers *writers;
-	int ret;
-
-	writers = kmalloc(sizeof(*writers), GFP_NOFS);
-	if (!writers)
-		return ERR_PTR(-ENOMEM);
-
-	ret = percpu_counter_init(&writers->counter, 0, GFP_NOFS);
-	if (ret < 0) {
-		kfree(writers);
-		return ERR_PTR(ret);
-	}
-
-	init_waitqueue_head(&writers->wait);
-	return writers;
-}
-
-static void
-btrfs_free_subvolume_writers(struct btrfs_subvolume_writers *writers)
-{
-	percpu_counter_destroy(&writers->counter);
-	kfree(writers);
-}
-
 static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 			 u64 objectid)
 {
@@ -1198,7 +1172,6 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	atomic_set(&root->log_writers, 0);
 	atomic_set(&root->log_batch, 0);
 	refcount_set(&root->refs, 1);
-	atomic_set(&root->will_be_snapshotted, 0);
 	atomic_set(&root->snapshot_force_cow, 0);
 	atomic_set(&root->nr_swapfiles, 0);
 	root->log_transid = 0;
@@ -1487,7 +1460,6 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_root *tree_root,
 int btrfs_init_fs_root(struct btrfs_root *root)
 {
 	int ret;
-	struct btrfs_subvolume_writers *writers;
 
 	root->free_ino_ctl = kzalloc(sizeof(*root->free_ino_ctl), GFP_NOFS);
 	root->free_ino_pinned = kzalloc(sizeof(*root->free_ino_pinned),
@@ -1497,12 +1469,8 @@ int btrfs_init_fs_root(struct btrfs_root *root)
 		goto fail;
 	}
 
-	writers = btrfs_alloc_subvolume_writers();
-	if (IS_ERR(writers)) {
-		ret = PTR_ERR(writers);
-		goto fail;
-	}
-	root->subv_writers = writers;
+
+	btrfs_drw_lock_init(&root->snapshot_lock);
 
 	btrfs_init_free_ino_ctl(root);
 	spin_lock_init(&root->ino_cache_lock);
@@ -3873,8 +3841,7 @@ void btrfs_free_fs_root(struct btrfs_root *root)
 	WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
 	if (root->anon_dev)
 		free_anon_bdev(root->anon_dev);
-	if (root->subv_writers)
-		btrfs_free_subvolume_writers(root->subv_writers);
+	btrfs_drw_lock_destroy(&root->snapshot_lock);
 	free_extent_buffer(root->node);
 	free_extent_buffer(root->commit_root);
 	kfree(root->free_ino_ctl);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5a11e4988243..3564bae0434d 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -11322,41 +11322,6 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
  * operations while snapshotting is ongoing and that cause the snapshot to be
  * inconsistent (writes followed by expanding truncates for example).
  */
-void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
-{
-	percpu_counter_dec(&root->subv_writers->counter);
-	cond_wake_up(&root->subv_writers->wait);
-}
-
-int btrfs_start_write_no_snapshotting(struct btrfs_root *root)
-{
-	if (atomic_read(&root->will_be_snapshotted))
-		return 0;
-
-	percpu_counter_inc(&root->subv_writers->counter);
-	/*
-	 * Make sure counter is updated before we check for snapshot creation.
-	 */
-	smp_mb();
-	if (atomic_read(&root->will_be_snapshotted)) {
-		btrfs_end_write_no_snapshotting(root);
-		return 0;
-	}
-	return 1;
-}
-
-void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
-{
-	while (true) {
-		int ret;
-
-		ret = btrfs_start_write_no_snapshotting(root);
-		if (ret)
-			break;
-		wait_var_event(&root->will_be_snapshotted,
-			       !atomic_read(&root->will_be_snapshotted));
-	}
-}
 
 void btrfs_mark_bg_unused(struct btrfs_block_group_cache *bg)
 {
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 5370152ea7e3..b9f01efff276 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -26,6 +26,7 @@
 #include "volumes.h"
 #include "qgroup.h"
 #include "compression.h"
+#include "drw_lock.h"
 
 static struct kmem_cache *btrfs_inode_defrag_cachep;
 /*
@@ -1554,8 +1555,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 	u64 num_bytes;
 	int ret;
 
-	ret = btrfs_start_write_no_snapshotting(root);
-	if (!ret)
+	if (!btrfs_drw_try_write_lock(&root->snapshot_lock))
 		return -EAGAIN;
 
 	lockstart = round_down(pos, fs_info->sectorsize);
@@ -1570,7 +1570,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 			NULL, NULL, NULL);
 	if (ret <= 0) {
 		ret = 0;
-		btrfs_end_write_no_snapshotting(root);
+		btrfs_drw_write_unlock(&root->snapshot_lock);
 	} else {
 		*write_bytes = min_t(size_t, *write_bytes ,
 				     num_bytes - pos + lockstart);
@@ -1675,7 +1675,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 						data_reserved, pos,
 						write_bytes);
 			else
-				btrfs_end_write_no_snapshotting(root);
+				btrfs_drw_write_unlock(&root->snapshot_lock);
 			break;
 		}
 
@@ -1769,7 +1769,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 
 		release_bytes = 0;
 		if (only_release_metadata)
-			btrfs_end_write_no_snapshotting(root);
+			btrfs_drw_write_unlock(&root->snapshot_lock);
 
 		if (only_release_metadata && copied > 0) {
 			lockstart = round_down(pos,
@@ -1799,7 +1799,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 
 	if (release_bytes) {
 		if (only_release_metadata) {
-			btrfs_end_write_no_snapshotting(root);
+			btrfs_drw_write_unlock(&root->snapshot_lock);
 			btrfs_delalloc_release_metadata(BTRFS_I(inode),
 					release_bytes, true);
 		} else {
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index f5e19ba27bdc..00118805ef00 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5102,16 +5102,16 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 		 * truncation, it must capture all writes that happened before
 		 * this truncation.
 		 */
-		btrfs_wait_for_snapshot_creation(root);
+		btrfs_drw_write_lock(&root->snapshot_lock);
 		ret = btrfs_cont_expand(inode, oldsize, newsize);
 		if (ret) {
-			btrfs_end_write_no_snapshotting(root);
+			btrfs_drw_write_unlock(&root->snapshot_lock);
 			return ret;
 		}
 
 		trans = btrfs_start_transaction(root, 1);
 		if (IS_ERR(trans)) {
-			btrfs_end_write_no_snapshotting(root);
+			btrfs_drw_write_unlock(&root->snapshot_lock);
 			return PTR_ERR(trans);
 		}
 
@@ -5119,7 +5119,7 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 		btrfs_ordered_update_i_size(inode, i_size_read(inode), NULL);
 		pagecache_isize_extended(inode, oldsize, newsize);
 		ret = btrfs_update_inode(trans, root, inode);
-		btrfs_end_write_no_snapshotting(root);
+		btrfs_drw_write_unlock(&root->snapshot_lock);
 		btrfs_end_transaction(trans);
 	} else {
 
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 803577d42518..e35f1b14d772 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -791,11 +791,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	 * possible. This is to avoid later writeback (running dealloc) to
 	 * fallback to COW mode and unexpectedly fail with ENOSPC.
 	 */
-	atomic_inc(&root->will_be_snapshotted);
-	smp_mb__after_atomic();
-	/* wait for no snapshot writes */
-	wait_event(root->subv_writers->wait,
-		   percpu_counter_sum(&root->subv_writers->counter) == 0);
+	btrfs_drw_read_lock(&root->snapshot_lock);
 
 	ret = btrfs_start_delalloc_snapshot(root);
 	if (ret)
@@ -875,8 +871,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 dec_and_free:
 	if (snapshot_force_cow)
 		atomic_dec(&root->snapshot_force_cow);
-	if (atomic_dec_and_test(&root->will_be_snapshotted))
-		wake_up_var(&root->will_be_snapshotted);
+	btrfs_drw_read_unlock(&root->snapshot_lock);
+
 free_pending:
 	kfree(pending_snapshot->root_item);
 	btrfs_free_path(pending_snapshot->path);
-- 
2.17.1

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Filipe Manana]

On Thu, Jun 6, 2019 at 2:55 PM Nikolay Borisov <nborisov@suse.com> wrote:
>
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/Makefile   |  2 +-
>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>  3 files changed, 95 insertions(+), 1 deletion(-)
>  create mode 100644 fs/btrfs/drw_lock.c
>  create mode 100644 fs/btrfs/drw_lock.h
>
> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> index ca693dd554e9..dc60127791e6 100644
> --- a/fs/btrfs/Makefile
> +++ b/fs/btrfs/Makefile
> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>            export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>            compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>            reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> -          uuid-tree.o props.o free-space-tree.o tree-checker.o
> +          uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>
>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> new file mode 100644
> index 000000000000..9681bf7544be
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.c
> @@ -0,0 +1,71 @@
> +#include "drw_lock.h"
> +#include "ctree.h"
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> +{
> +       atomic_set(&lock->readers, 0);
> +       percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> +       init_waitqueue_head(&lock->pending_readers);
> +       init_waitqueue_head(&lock->pending_writers);
> +}
> +
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> +{
> +       percpu_counter_destroy(&lock->writers);
> +}
> +
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> +{
> +       if (atomic_read(&lock->readers))
> +               return false;
> +
> +       percpu_counter_inc(&lock->writers);
> +
> +       /*
> +        * Ensure writers count is updated before we check for
> +        * pending readers
> +        */
> +       smp_mb();
> +       if (atomic_read(&lock->readers)) {
> +               btrfs_drw_read_unlock(lock);

Should be btrfs_drw_write_unlock(lock)

> +               return false;
> +       }
> +
> +       return true;
> +}
> +
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> +{
> +       while(true) {

Missing space after 'while'.

Thanks.

> +               if (btrfs_drw_try_write_lock(lock))
> +                       return;
> +               wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> +       }
> +}
> +
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> +{
> +       percpu_counter_dec(&lock->writers);
> +       cond_wake_up(&lock->pending_readers);
> +}
> +
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> +{
> +       atomic_inc(&lock->readers);
> +       smp_mb__after_atomic();
> +
> +       wait_event(lock->pending_readers,
> +                  percpu_counter_sum(&lock->writers) == 0);
> +}
> +
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> +{
> +       /*
> +        * Atomic RMW operations imply full barrier, so woken up writers
> +        * are guaranteed to see the decrement
> +        */
> +       if (atomic_dec_and_test(&lock->readers))
> +               wake_up(&lock->pending_writers);
> +}
> +
> +
> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> new file mode 100644
> index 000000000000..baff59561c06
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.h
> @@ -0,0 +1,23 @@
> +#ifndef BTRFS_DRW_LOCK_H
> +#define BTRFS_DRW_LOCK_H
> +
> +#include <linux/atomic.h>
> +#include <linux/wait.h>
> +#include <linux/percpu_counter.h>
> +
> +struct btrfs_drw_lock {
> +       atomic_t readers;
> +       struct percpu_counter writers;
> +       wait_queue_head_t pending_writers;
> +       wait_queue_head_t pending_readers;
> +};
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> +
> +#endif
> --
> 2.17.1
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH 2/2] btrfs: convert snapshot/nocow exlcusion to drw lock

[Filipe Manana]

On Thu, Jun 6, 2019 at 2:54 PM Nikolay Borisov <nborisov@suse.com> wrote:
>
> This patch removes all haphazard code implementing nocow writers
> exclusion from pending snapshot creation and switches to using the drw
> lock to ensure this invariant still holds. "Readers" are snapshot
> creators from create_snapshot and 'writers' are nocow writers from
> buffered write path or btrfs_setsize. This locking scheme allows for
> multiple snapshots to happen while any nocow writers are blocked, since
> writes to page cache in the nocow path will make snapshots inconsistent.
>
> So for performance reasons we'd like to have the ability to run multiple
> concurrent snapshots and also favors readers in this case. And in case
> there aren't pending snapshots (which will be the majority of the cases)
> we rely on the percpu's writers counter to avoid cacheline contention.
>
> The main gain from using the drw is it's now a lot easier to reason about
> the guarantees of the locking scheme and whether there is some silent
> breakage lurking.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/ctree.h       | 10 +++-------
>  fs/btrfs/disk-io.c     | 39 +++------------------------------------
>  fs/btrfs/extent-tree.c | 35 -----------------------------------
>  fs/btrfs/file.c        | 12 ++++++------
>  fs/btrfs/inode.c       |  8 ++++----
>  fs/btrfs/ioctl.c       | 10 +++-------
>  6 files changed, 19 insertions(+), 95 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index a66ed58058d9..fa8a2e15c77c 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -32,6 +32,7 @@
>  #include "extent_io.h"
>  #include "extent_map.h"
>  #include "async-thread.h"
> +#include "drw_lock.h"
>
>  struct btrfs_trans_handle;
>  struct btrfs_transaction;
> @@ -1174,11 +1175,6 @@ static inline struct btrfs_fs_info *btrfs_sb(struct super_block *sb)
>         return sb->s_fs_info;
>  }
>
> -struct btrfs_subvolume_writers {
> -       struct percpu_counter   counter;
> -       wait_queue_head_t       wait;
> -};
> -
>  /*
>   * The state of btrfs root
>   */
> @@ -1350,8 +1346,8 @@ struct btrfs_root {
>          * root_item_lock.
>          */
>         int dedupe_in_progress;
> -       struct btrfs_subvolume_writers *subv_writers;
> -       atomic_t will_be_snapshotted;
> +       struct btrfs_drw_lock snapshot_lock;
> +
>         atomic_t snapshot_force_cow;
>
>         /* For qgroup metadata reserved space */
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 05f215b4d060..ece45e606846 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1125,32 +1125,6 @@ void btrfs_clean_tree_block(struct extent_buffer *buf)
>         }
>  }
>
> -static struct btrfs_subvolume_writers *btrfs_alloc_subvolume_writers(void)
> -{
> -       struct btrfs_subvolume_writers *writers;
> -       int ret;
> -
> -       writers = kmalloc(sizeof(*writers), GFP_NOFS);
> -       if (!writers)
> -               return ERR_PTR(-ENOMEM);
> -
> -       ret = percpu_counter_init(&writers->counter, 0, GFP_NOFS);
> -       if (ret < 0) {
> -               kfree(writers);
> -               return ERR_PTR(ret);
> -       }
> -
> -       init_waitqueue_head(&writers->wait);
> -       return writers;
> -}
> -
> -static void
> -btrfs_free_subvolume_writers(struct btrfs_subvolume_writers *writers)
> -{
> -       percpu_counter_destroy(&writers->counter);
> -       kfree(writers);
> -}
> -
>  static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
>                          u64 objectid)
>  {
> @@ -1198,7 +1172,6 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
>         atomic_set(&root->log_writers, 0);
>         atomic_set(&root->log_batch, 0);
>         refcount_set(&root->refs, 1);
> -       atomic_set(&root->will_be_snapshotted, 0);
>         atomic_set(&root->snapshot_force_cow, 0);
>         atomic_set(&root->nr_swapfiles, 0);
>         root->log_transid = 0;
> @@ -1487,7 +1460,6 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_root *tree_root,
>  int btrfs_init_fs_root(struct btrfs_root *root)
>  {
>         int ret;
> -       struct btrfs_subvolume_writers *writers;
>
>         root->free_ino_ctl = kzalloc(sizeof(*root->free_ino_ctl), GFP_NOFS);
>         root->free_ino_pinned = kzalloc(sizeof(*root->free_ino_pinned),
> @@ -1497,12 +1469,8 @@ int btrfs_init_fs_root(struct btrfs_root *root)
>                 goto fail;
>         }
>
> -       writers = btrfs_alloc_subvolume_writers();
> -       if (IS_ERR(writers)) {
> -               ret = PTR_ERR(writers);
> -               goto fail;
> -       }
> -       root->subv_writers = writers;
> +
> +       btrfs_drw_lock_init(&root->snapshot_lock);

So that does a GFP_KERNEL allocation, where the old code did a NOFS one.
You have to setup a nofs context before calling this new function,
because this code path, at least when called from the backref walking
code,
can be under a transaction context, in which case if the allocation
triggers reclaim we can deadlock.

Thanks.

>
>         btrfs_init_free_ino_ctl(root);
>         spin_lock_init(&root->ino_cache_lock);
> @@ -3873,8 +3841,7 @@ void btrfs_free_fs_root(struct btrfs_root *root)
>         WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
>         if (root->anon_dev)
>                 free_anon_bdev(root->anon_dev);
> -       if (root->subv_writers)
> -               btrfs_free_subvolume_writers(root->subv_writers);
> +       btrfs_drw_lock_destroy(&root->snapshot_lock);
>         free_extent_buffer(root->node);
>         free_extent_buffer(root->commit_root);
>         kfree(root->free_ino_ctl);
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 5a11e4988243..3564bae0434d 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -11322,41 +11322,6 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
>   * operations while snapshotting is ongoing and that cause the snapshot to be
>   * inconsistent (writes followed by expanding truncates for example).
>   */
> -void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
> -{
> -       percpu_counter_dec(&root->subv_writers->counter);
> -       cond_wake_up(&root->subv_writers->wait);
> -}
> -
> -int btrfs_start_write_no_snapshotting(struct btrfs_root *root)
> -{
> -       if (atomic_read(&root->will_be_snapshotted))
> -               return 0;
> -
> -       percpu_counter_inc(&root->subv_writers->counter);
> -       /*
> -        * Make sure counter is updated before we check for snapshot creation.
> -        */
> -       smp_mb();
> -       if (atomic_read(&root->will_be_snapshotted)) {
> -               btrfs_end_write_no_snapshotting(root);
> -               return 0;
> -       }
> -       return 1;
> -}
> -
> -void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
> -{
> -       while (true) {
> -               int ret;
> -
> -               ret = btrfs_start_write_no_snapshotting(root);
> -               if (ret)
> -                       break;
> -               wait_var_event(&root->will_be_snapshotted,
> -                              !atomic_read(&root->will_be_snapshotted));
> -       }
> -}
>
>  void btrfs_mark_bg_unused(struct btrfs_block_group_cache *bg)
>  {
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 5370152ea7e3..b9f01efff276 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -26,6 +26,7 @@
>  #include "volumes.h"
>  #include "qgroup.h"
>  #include "compression.h"
> +#include "drw_lock.h"
>
>  static struct kmem_cache *btrfs_inode_defrag_cachep;
>  /*
> @@ -1554,8 +1555,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
>         u64 num_bytes;
>         int ret;
>
> -       ret = btrfs_start_write_no_snapshotting(root);
> -       if (!ret)
> +       if (!btrfs_drw_try_write_lock(&root->snapshot_lock))
>                 return -EAGAIN;
>
>         lockstart = round_down(pos, fs_info->sectorsize);
> @@ -1570,7 +1570,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
>                         NULL, NULL, NULL);
>         if (ret <= 0) {
>                 ret = 0;
> -               btrfs_end_write_no_snapshotting(root);
> +               btrfs_drw_write_unlock(&root->snapshot_lock);
>         } else {
>                 *write_bytes = min_t(size_t, *write_bytes ,
>                                      num_bytes - pos + lockstart);
> @@ -1675,7 +1675,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
>                                                 data_reserved, pos,
>                                                 write_bytes);
>                         else
> -                               btrfs_end_write_no_snapshotting(root);
> +                               btrfs_drw_write_unlock(&root->snapshot_lock);
>                         break;
>                 }
>
> @@ -1769,7 +1769,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
>
>                 release_bytes = 0;
>                 if (only_release_metadata)
> -                       btrfs_end_write_no_snapshotting(root);
> +                       btrfs_drw_write_unlock(&root->snapshot_lock);
>
>                 if (only_release_metadata && copied > 0) {
>                         lockstart = round_down(pos,
> @@ -1799,7 +1799,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
>
>         if (release_bytes) {
>                 if (only_release_metadata) {
> -                       btrfs_end_write_no_snapshotting(root);
> +                       btrfs_drw_write_unlock(&root->snapshot_lock);
>                         btrfs_delalloc_release_metadata(BTRFS_I(inode),
>                                         release_bytes, true);
>                 } else {
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index f5e19ba27bdc..00118805ef00 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -5102,16 +5102,16 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
>                  * truncation, it must capture all writes that happened before
>                  * this truncation.
>                  */
> -               btrfs_wait_for_snapshot_creation(root);
> +               btrfs_drw_write_lock(&root->snapshot_lock);
>                 ret = btrfs_cont_expand(inode, oldsize, newsize);
>                 if (ret) {
> -                       btrfs_end_write_no_snapshotting(root);
> +                       btrfs_drw_write_unlock(&root->snapshot_lock);
>                         return ret;
>                 }
>
>                 trans = btrfs_start_transaction(root, 1);
>                 if (IS_ERR(trans)) {
> -                       btrfs_end_write_no_snapshotting(root);
> +                       btrfs_drw_write_unlock(&root->snapshot_lock);
>                         return PTR_ERR(trans);
>                 }
>
> @@ -5119,7 +5119,7 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
>                 btrfs_ordered_update_i_size(inode, i_size_read(inode), NULL);
>                 pagecache_isize_extended(inode, oldsize, newsize);
>                 ret = btrfs_update_inode(trans, root, inode);
> -               btrfs_end_write_no_snapshotting(root);
> +               btrfs_drw_write_unlock(&root->snapshot_lock);
>                 btrfs_end_transaction(trans);
>         } else {
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 803577d42518..e35f1b14d772 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -791,11 +791,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
>          * possible. This is to avoid later writeback (running dealloc) to
>          * fallback to COW mode and unexpectedly fail with ENOSPC.
>          */
> -       atomic_inc(&root->will_be_snapshotted);
> -       smp_mb__after_atomic();
> -       /* wait for no snapshot writes */
> -       wait_event(root->subv_writers->wait,
> -                  percpu_counter_sum(&root->subv_writers->counter) == 0);
> +       btrfs_drw_read_lock(&root->snapshot_lock);
>
>         ret = btrfs_start_delalloc_snapshot(root);
>         if (ret)
> @@ -875,8 +871,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
>  dec_and_free:
>         if (snapshot_force_cow)
>                 atomic_dec(&root->snapshot_force_cow);
> -       if (atomic_dec_and_test(&root->will_be_snapshotted))
> -               wake_up_var(&root->will_be_snapshotted);
> +       btrfs_drw_read_unlock(&root->snapshot_lock);
> +
>  free_pending:
>         kfree(pending_snapshot->root_item);
>         btrfs_free_path(pending_snapshot->path);
> --
> 2.17.1
>


--
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Paul E. McKenney]

On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

A preliminary question...

What prevents the following sequence of events from happening?

o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
	which sees that lock->readers is zero and thus executes
	percpu_counter_inc(&lock->writers).

o	btrfs_drw_read_lock() increments lock->readers, does the
	smp_mb__after_atomic(), and then does the wait_event().
	Because btrfs_drw_try_write_lock() incremented its CPU's
	lock->writers, the sum is the value one, so it blocks.

o	btrfs_drw_try_write_lock() checks lock->readers, sees that
	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
	(which decrements the current CPU's counter, so that a future
	sum would get zero), and returns false.

o	btrfs_drw_write_lock() therefore does its wait_event().
	Because lock->readers is nonzero, it blocks.

o	Both tasks are now blocked.  In the absence of future calls
	to these functions (and perhaps even given such future calls),
	we have deadlock.

So what am I missing here?

							Thanx, Paul

> ---
>  fs/btrfs/Makefile   |  2 +-
>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>  3 files changed, 95 insertions(+), 1 deletion(-)
>  create mode 100644 fs/btrfs/drw_lock.c
>  create mode 100644 fs/btrfs/drw_lock.h
> 
> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> index ca693dd554e9..dc60127791e6 100644
> --- a/fs/btrfs/Makefile
> +++ b/fs/btrfs/Makefile
> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>  
>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> new file mode 100644
> index 000000000000..9681bf7544be
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.c
> @@ -0,0 +1,71 @@
> +#include "drw_lock.h"
> +#include "ctree.h"
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> +{
> +	atomic_set(&lock->readers, 0);
> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> +	init_waitqueue_head(&lock->pending_readers);
> +	init_waitqueue_head(&lock->pending_writers);
> +}
> +
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> +{
> +	percpu_counter_destroy(&lock->writers);
> +}
> +
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> +{
> +	if (atomic_read(&lock->readers))
> +		return false;
> +
> +	percpu_counter_inc(&lock->writers);
> +
> +	/*
> +	 * Ensure writers count is updated before we check for
> +	 * pending readers
> +	 */
> +	smp_mb();
> +	if (atomic_read(&lock->readers)) {
> +		btrfs_drw_read_unlock(lock);
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> +{
> +	while(true) {
> +		if (btrfs_drw_try_write_lock(lock))
> +			return;
> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> +	}
> +}
> +
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> +{
> +	percpu_counter_dec(&lock->writers);
> +	cond_wake_up(&lock->pending_readers);
> +}
> +
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> +{
> +	atomic_inc(&lock->readers);
> +	smp_mb__after_atomic();
> +
> +	wait_event(lock->pending_readers,
> +		   percpu_counter_sum(&lock->writers) == 0);
> +}
> +
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> +{
> +	/*
> +	 * Atomic RMW operations imply full barrier, so woken up writers
> +	 * are guaranteed to see the decrement
> +	 */
> +	if (atomic_dec_and_test(&lock->readers))
> +		wake_up(&lock->pending_writers);
> +}
> +
> +
> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> new file mode 100644
> index 000000000000..baff59561c06
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.h
> @@ -0,0 +1,23 @@
> +#ifndef BTRFS_DRW_LOCK_H
> +#define BTRFS_DRW_LOCK_H
> +
> +#include <linux/atomic.h>
> +#include <linux/wait.h>
> +#include <linux/percpu_counter.h>
> +
> +struct btrfs_drw_lock {
> +	atomic_t readers;
> +	struct percpu_counter writers;
> +	wait_queue_head_t pending_writers;
> +	wait_queue_head_t pending_readers;
> +};
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> +
> +#endif
> -- 
> 2.17.1
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
>> to have multiple readers or multiple writers but not multiple readers
>> and writers holding it concurrently. The code is factored out from
>> the existing open-coded locking scheme used to exclude pending
>> snapshots from nocow writers and vice-versa. Current implementation
>> actually favors Readers (that is snapshot creaters) to writers (nocow
>> writers of the filesystem).
>>
>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> 
> A preliminary question...
> 
> What prevents the following sequence of events from happening?
> 
> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
> 	which sees that lock->readers is zero and thus executes
> 	percpu_counter_inc(&lock->writers).
> 
> o	btrfs_drw_read_lock() increments lock->readers, does the
> 	smp_mb__after_atomic(), and then does the wait_event().
> 	Because btrfs_drw_try_write_lock() incremented its CPU's
> 	lock->writers, the sum is the value one, so it blocks.
> 
> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
> 	(which decrements the current CPU's counter, so that a future
> 	sum would get zero), and returns false.

btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
already pointed that out and I've fixed it.

The idea here is that if a reader came after we've incremented out
percpu counter then it would have blocked, the writer would see that and
invoke btrfs_drw_write_unlock which will decrement the percpu counter
and will wakeup the reader that is now blocked on pending_readers.

> 
> o	btrfs_drw_write_lock() therefore does its wait_event().
> 	Because lock->readers is nonzero, it blocks.
> 
> o	Both tasks are now blocked.  In the absence of future calls
> 	to these functions (and perhaps even given such future calls),
> 	we have deadlock.
> 
> So what am I missing here?
> 
> 							Thanx, Paul
> 
>> ---
>>  fs/btrfs/Makefile   |  2 +-
>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>>  3 files changed, 95 insertions(+), 1 deletion(-)
>>  create mode 100644 fs/btrfs/drw_lock.c
>>  create mode 100644 fs/btrfs/drw_lock.h
>>
>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
>> index ca693dd554e9..dc60127791e6 100644
>> --- a/fs/btrfs/Makefile
>> +++ b/fs/btrfs/Makefile
>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>>  
>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
>> new file mode 100644
>> index 000000000000..9681bf7544be
>> --- /dev/null
>> +++ b/fs/btrfs/drw_lock.c
>> @@ -0,0 +1,71 @@
>> +#include "drw_lock.h"
>> +#include "ctree.h"
>> +
>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
>> +{
>> +	atomic_set(&lock->readers, 0);
>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
>> +	init_waitqueue_head(&lock->pending_readers);
>> +	init_waitqueue_head(&lock->pending_writers);
>> +}
>> +
>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
>> +{
>> +	percpu_counter_destroy(&lock->writers);
>> +}
>> +
>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
>> +{
>> +	if (atomic_read(&lock->readers))
>> +		return false;
>> +
>> +	percpu_counter_inc(&lock->writers);
>> +
>> +	/*
>> +	 * Ensure writers count is updated before we check for
>> +	 * pending readers
>> +	 */
>> +	smp_mb();
>> +	if (atomic_read(&lock->readers)) {
>> +		btrfs_drw_read_unlock(lock);
>> +		return false;
>> +	}
>> +
>> +	return true;
>> +}
>> +
>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
>> +{
>> +	while(true) {
>> +		if (btrfs_drw_try_write_lock(lock))
>> +			return;
>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
>> +	}
>> +}
>> +
>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
>> +{
>> +	percpu_counter_dec(&lock->writers);
>> +	cond_wake_up(&lock->pending_readers);
>> +}
>> +
>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
>> +{
>> +	atomic_inc(&lock->readers);
>> +	smp_mb__after_atomic();
>> +
>> +	wait_event(lock->pending_readers,
>> +		   percpu_counter_sum(&lock->writers) == 0);
>> +}
>> +
>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
>> +{
>> +	/*
>> +	 * Atomic RMW operations imply full barrier, so woken up writers
>> +	 * are guaranteed to see the decrement
>> +	 */
>> +	if (atomic_dec_and_test(&lock->readers))
>> +		wake_up(&lock->pending_writers);
>> +}
>> +
>> +
>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
>> new file mode 100644
>> index 000000000000..baff59561c06
>> --- /dev/null
>> +++ b/fs/btrfs/drw_lock.h
>> @@ -0,0 +1,23 @@
>> +#ifndef BTRFS_DRW_LOCK_H
>> +#define BTRFS_DRW_LOCK_H
>> +
>> +#include <linux/atomic.h>
>> +#include <linux/wait.h>
>> +#include <linux/percpu_counter.h>
>> +
>> +struct btrfs_drw_lock {
>> +	atomic_t readers;
>> +	struct percpu_counter writers;
>> +	wait_queue_head_t pending_writers;
>> +	wait_queue_head_t pending_readers;
>> +};
>> +
>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
>> +
>> +#endif
>> -- 
>> 2.17.1
>>
> 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Paul E. McKenney]

On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
> 
> 
> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
> > On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> >> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> >> to have multiple readers or multiple writers but not multiple readers
> >> and writers holding it concurrently. The code is factored out from
> >> the existing open-coded locking scheme used to exclude pending
> >> snapshots from nocow writers and vice-versa. Current implementation
> >> actually favors Readers (that is snapshot creaters) to writers (nocow
> >> writers of the filesystem).
> >>
> >> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> > 
> > A preliminary question...
> > 
> > What prevents the following sequence of events from happening?
> > 
> > o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
> > 	which sees that lock->readers is zero and thus executes
> > 	percpu_counter_inc(&lock->writers).
> > 
> > o	btrfs_drw_read_lock() increments lock->readers, does the
> > 	smp_mb__after_atomic(), and then does the wait_event().
> > 	Because btrfs_drw_try_write_lock() incremented its CPU's
> > 	lock->writers, the sum is the value one, so it blocks.
> > 
> > o	btrfs_drw_try_write_lock() checks lock->readers, sees that
> > 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
> > 	(which decrements the current CPU's counter, so that a future
> > 	sum would get zero), and returns false.
> 
> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
> already pointed that out and I've fixed it.

Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?

> The idea here is that if a reader came after we've incremented out
> percpu counter then it would have blocked, the writer would see that and
> invoke btrfs_drw_write_unlock which will decrement the percpu counter
> and will wakeup the reader that is now blocked on pending_readers.

OK, I will await your next version.

							Thanx, Paul

> > o	btrfs_drw_write_lock() therefore does its wait_event().
> > 	Because lock->readers is nonzero, it blocks.
> > 
> > o	Both tasks are now blocked.  In the absence of future calls
> > 	to these functions (and perhaps even given such future calls),
> > 	we have deadlock.
> > 
> > So what am I missing here?
> > 
> > 							Thanx, Paul
> > 
> >> ---
> >>  fs/btrfs/Makefile   |  2 +-
> >>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
> >>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
> >>  3 files changed, 95 insertions(+), 1 deletion(-)
> >>  create mode 100644 fs/btrfs/drw_lock.c
> >>  create mode 100644 fs/btrfs/drw_lock.h
> >>
> >> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> >> index ca693dd554e9..dc60127791e6 100644
> >> --- a/fs/btrfs/Makefile
> >> +++ b/fs/btrfs/Makefile
> >> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
> >>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
> >>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
> >>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> >> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
> >> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
> >>  
> >>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
> >>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> >> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> >> new file mode 100644
> >> index 000000000000..9681bf7544be
> >> --- /dev/null
> >> +++ b/fs/btrfs/drw_lock.c
> >> @@ -0,0 +1,71 @@
> >> +#include "drw_lock.h"
> >> +#include "ctree.h"
> >> +
> >> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> >> +{
> >> +	atomic_set(&lock->readers, 0);
> >> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> >> +	init_waitqueue_head(&lock->pending_readers);
> >> +	init_waitqueue_head(&lock->pending_writers);
> >> +}
> >> +
> >> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> >> +{
> >> +	percpu_counter_destroy(&lock->writers);
> >> +}
> >> +
> >> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> >> +{
> >> +	if (atomic_read(&lock->readers))
> >> +		return false;
> >> +
> >> +	percpu_counter_inc(&lock->writers);
> >> +
> >> +	/*
> >> +	 * Ensure writers count is updated before we check for
> >> +	 * pending readers
> >> +	 */
> >> +	smp_mb();
> >> +	if (atomic_read(&lock->readers)) {
> >> +		btrfs_drw_read_unlock(lock);
> >> +		return false;
> >> +	}
> >> +
> >> +	return true;
> >> +}
> >> +
> >> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> >> +{
> >> +	while(true) {
> >> +		if (btrfs_drw_try_write_lock(lock))
> >> +			return;
> >> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> >> +	}
> >> +}
> >> +
> >> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> >> +{
> >> +	percpu_counter_dec(&lock->writers);
> >> +	cond_wake_up(&lock->pending_readers);
> >> +}
> >> +
> >> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> >> +{
> >> +	atomic_inc(&lock->readers);
> >> +	smp_mb__after_atomic();
> >> +
> >> +	wait_event(lock->pending_readers,
> >> +		   percpu_counter_sum(&lock->writers) == 0);
> >> +}
> >> +
> >> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> >> +{
> >> +	/*
> >> +	 * Atomic RMW operations imply full barrier, so woken up writers
> >> +	 * are guaranteed to see the decrement
> >> +	 */
> >> +	if (atomic_dec_and_test(&lock->readers))
> >> +		wake_up(&lock->pending_writers);
> >> +}
> >> +
> >> +
> >> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> >> new file mode 100644
> >> index 000000000000..baff59561c06
> >> --- /dev/null
> >> +++ b/fs/btrfs/drw_lock.h
> >> @@ -0,0 +1,23 @@
> >> +#ifndef BTRFS_DRW_LOCK_H
> >> +#define BTRFS_DRW_LOCK_H
> >> +
> >> +#include <linux/atomic.h>
> >> +#include <linux/wait.h>
> >> +#include <linux/percpu_counter.h>
> >> +
> >> +struct btrfs_drw_lock {
> >> +	atomic_t readers;
> >> +	struct percpu_counter writers;
> >> +	wait_queue_head_t pending_writers;
> >> +	wait_queue_head_t pending_readers;
> >> +};
> >> +
> >> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> >> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> >> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> >> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> >> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> >> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> >> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> >> +
> >> +#endif
> >> -- 
> >> 2.17.1
> >>
> > 
> > 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

On 8.06.19 г. 18:13 ч., Paul E. McKenney wrote:
> On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
>>
>>
>> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
>>> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
>>>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
>>>> to have multiple readers or multiple writers but not multiple readers
>>>> and writers holding it concurrently. The code is factored out from
>>>> the existing open-coded locking scheme used to exclude pending
>>>> snapshots from nocow writers and vice-versa. Current implementation
>>>> actually favors Readers (that is snapshot creaters) to writers (nocow
>>>> writers of the filesystem).
>>>>
>>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>>>
>>> A preliminary question...
>>>
>>> What prevents the following sequence of events from happening?
>>>
>>> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
>>> 	which sees that lock->readers is zero and thus executes
>>> 	percpu_counter_inc(&lock->writers).
>>>
>>> o	btrfs_drw_read_lock() increments lock->readers, does the
>>> 	smp_mb__after_atomic(), and then does the wait_event().
>>> 	Because btrfs_drw_try_write_lock() incremented its CPU's
>>> 	lock->writers, the sum is the value one, so it blocks.
>>>
>>> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
>>> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
>>> 	(which decrements the current CPU's counter, so that a future
>>> 	sum would get zero), and returns false.
>>
>> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
>> already pointed that out and I've fixed it.
> 
> Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?

At the moment - nothing. I rely on the fact that the original code I
extracted that from is bug-free (ha-ha). So perhahps hooking up
locktorture seems like a good suggestion. From a quick look I guess I
could mostly model that lock against the rwsem. The question is how do I
model the trylock semantics as well as the "double" part?

> 
>> The idea here is that if a reader came after we've incremented out
>> percpu counter then it would have blocked, the writer would see that and
>> invoke btrfs_drw_write_unlock which will decrement the percpu counter
>> and will wakeup the reader that is now blocked on pending_readers.
> 
> OK, I will await your next version.
> 
> 							Thanx, Paul
> 
>>> o	btrfs_drw_write_lock() therefore does its wait_event().
>>> 	Because lock->readers is nonzero, it blocks.
>>>
>>> o	Both tasks are now blocked.  In the absence of future calls
>>> 	to these functions (and perhaps even given such future calls),
>>> 	we have deadlock.
>>>
>>> So what am I missing here?
>>>
>>> 							Thanx, Paul
>>>
>>>> ---
>>>>  fs/btrfs/Makefile   |  2 +-
>>>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>>>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>>>>  3 files changed, 95 insertions(+), 1 deletion(-)
>>>>  create mode 100644 fs/btrfs/drw_lock.c
>>>>  create mode 100644 fs/btrfs/drw_lock.h
>>>>
>>>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
>>>> index ca693dd554e9..dc60127791e6 100644
>>>> --- a/fs/btrfs/Makefile
>>>> +++ b/fs/btrfs/Makefile
>>>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>>>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>>>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>>>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
>>>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
>>>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>>>>  
>>>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>>>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
>>>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
>>>> new file mode 100644
>>>> index 000000000000..9681bf7544be
>>>> --- /dev/null
>>>> +++ b/fs/btrfs/drw_lock.c
>>>> @@ -0,0 +1,71 @@
>>>> +#include "drw_lock.h"
>>>> +#include "ctree.h"
>>>> +
>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	atomic_set(&lock->readers, 0);
>>>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
>>>> +	init_waitqueue_head(&lock->pending_readers);
>>>> +	init_waitqueue_head(&lock->pending_writers);
>>>> +}
>>>> +
>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	percpu_counter_destroy(&lock->writers);
>>>> +}
>>>> +
>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	if (atomic_read(&lock->readers))
>>>> +		return false;
>>>> +
>>>> +	percpu_counter_inc(&lock->writers);
>>>> +
>>>> +	/*
>>>> +	 * Ensure writers count is updated before we check for
>>>> +	 * pending readers
>>>> +	 */
>>>> +	smp_mb();
>>>> +	if (atomic_read(&lock->readers)) {
>>>> +		btrfs_drw_read_unlock(lock);
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	return true;
>>>> +}
>>>> +
>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	while(true) {
>>>> +		if (btrfs_drw_try_write_lock(lock))
>>>> +			return;
>>>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
>>>> +	}
>>>> +}
>>>> +
>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	percpu_counter_dec(&lock->writers);
>>>> +	cond_wake_up(&lock->pending_readers);
>>>> +}
>>>> +
>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	atomic_inc(&lock->readers);
>>>> +	smp_mb__after_atomic();
>>>> +
>>>> +	wait_event(lock->pending_readers,
>>>> +		   percpu_counter_sum(&lock->writers) == 0);
>>>> +}
>>>> +
>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
>>>> +{
>>>> +	/*
>>>> +	 * Atomic RMW operations imply full barrier, so woken up writers
>>>> +	 * are guaranteed to see the decrement
>>>> +	 */
>>>> +	if (atomic_dec_and_test(&lock->readers))
>>>> +		wake_up(&lock->pending_writers);
>>>> +}
>>>> +
>>>> +
>>>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
>>>> new file mode 100644
>>>> index 000000000000..baff59561c06
>>>> --- /dev/null
>>>> +++ b/fs/btrfs/drw_lock.h
>>>> @@ -0,0 +1,23 @@
>>>> +#ifndef BTRFS_DRW_LOCK_H
>>>> +#define BTRFS_DRW_LOCK_H
>>>> +
>>>> +#include <linux/atomic.h>
>>>> +#include <linux/wait.h>
>>>> +#include <linux/percpu_counter.h>
>>>> +
>>>> +struct btrfs_drw_lock {
>>>> +	atomic_t readers;
>>>> +	struct percpu_counter writers;
>>>> +	wait_queue_head_t pending_writers;
>>>> +	wait_queue_head_t pending_readers;
>>>> +};
>>>> +
>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
>>>> +
>>>> +#endif
>>>> -- 
>>>> 2.17.1
>>>>
>>>
>>>
>>
> 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Paul E. McKenney]

On Sat, Jun 08, 2019 at 06:44:17PM +0300, Nikolay Borisov wrote:
> On 8.06.19 г. 18:13 ч., Paul E. McKenney wrote:
> > On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
> >> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
> >>> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> >>>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> >>>> to have multiple readers or multiple writers but not multiple readers
> >>>> and writers holding it concurrently. The code is factored out from
> >>>> the existing open-coded locking scheme used to exclude pending
> >>>> snapshots from nocow writers and vice-versa. Current implementation
> >>>> actually favors Readers (that is snapshot creaters) to writers (nocow
> >>>> writers of the filesystem).
> >>>>
> >>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> >>>
> >>> A preliminary question...
> >>>
> >>> What prevents the following sequence of events from happening?
> >>>
> >>> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
> >>> 	which sees that lock->readers is zero and thus executes
> >>> 	percpu_counter_inc(&lock->writers).
> >>>
> >>> o	btrfs_drw_read_lock() increments lock->readers, does the
> >>> 	smp_mb__after_atomic(), and then does the wait_event().
> >>> 	Because btrfs_drw_try_write_lock() incremented its CPU's
> >>> 	lock->writers, the sum is the value one, so it blocks.
> >>>
> >>> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
> >>> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
> >>> 	(which decrements the current CPU's counter, so that a future
> >>> 	sum would get zero), and returns false.
> >>
> >> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
> >> already pointed that out and I've fixed it.
> > 
> > Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?

Right...  Make that kernel/locking/locktorture.c

> At the moment - nothing. I rely on the fact that the original code I
> extracted that from is bug-free (ha-ha). So perhahps hooking up
> locktorture seems like a good suggestion. From a quick look I guess I
> could mostly model that lock against the rwsem. The question is how do I
> model the trylock semantics as well as the "double" part?

Implementing a correct synchronization primitive is like committing the
perfect crime.  There are at least 50 things that can go wrong, and if
you are a highly experienced genius, you -might- be able to anticipate
and handle 25 of them.  (With apologies to any Kathleen Turner fans who
might still be alive.)  Please note that this still applies to code
ported from somewhere else because different environments likely have
different assumptions and properties.

Therefore, heavy-duty stress testing is not optional.  In fact, formal
verification is becoming non-optional as well -- please see Catalin
Marinas's work on verifying the Linux kernel's queued spinlock for
an example.

You are right, current locktorture would get upset about having concurrent
writers.  To teach locktorture about this, I suggest adding a flag to
the lock_torture_ops structure named something like concurrent_write,
but hopefully shorter.  Then this flag can be used to disable the "only
one writer" check in lock_torture_writer().

Seem reasonable?

							Thanx, Paul

> >> The idea here is that if a reader came after we've incremented out
> >> percpu counter then it would have blocked, the writer would see that and
> >> invoke btrfs_drw_write_unlock which will decrement the percpu counter
> >> and will wakeup the reader that is now blocked on pending_readers.
> > 
> > OK, I will await your next version.
> > 
> > 							Thanx, Paul
> > 
> >>> o	btrfs_drw_write_lock() therefore does its wait_event().
> >>> 	Because lock->readers is nonzero, it blocks.
> >>>
> >>> o	Both tasks are now blocked.  In the absence of future calls
> >>> 	to these functions (and perhaps even given such future calls),
> >>> 	we have deadlock.
> >>>
> >>> So what am I missing here?
> >>>
> >>> 							Thanx, Paul
> >>>
> >>>> ---
> >>>>  fs/btrfs/Makefile   |  2 +-
> >>>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
> >>>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
> >>>>  3 files changed, 95 insertions(+), 1 deletion(-)
> >>>>  create mode 100644 fs/btrfs/drw_lock.c
> >>>>  create mode 100644 fs/btrfs/drw_lock.h
> >>>>
> >>>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> >>>> index ca693dd554e9..dc60127791e6 100644
> >>>> --- a/fs/btrfs/Makefile
> >>>> +++ b/fs/btrfs/Makefile
> >>>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
> >>>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
> >>>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
> >>>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> >>>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
> >>>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
> >>>>  
> >>>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
> >>>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> >>>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> >>>> new file mode 100644
> >>>> index 000000000000..9681bf7544be
> >>>> --- /dev/null
> >>>> +++ b/fs/btrfs/drw_lock.c
> >>>> @@ -0,0 +1,71 @@
> >>>> +#include "drw_lock.h"
> >>>> +#include "ctree.h"
> >>>> +
> >>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	atomic_set(&lock->readers, 0);
> >>>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> >>>> +	init_waitqueue_head(&lock->pending_readers);
> >>>> +	init_waitqueue_head(&lock->pending_writers);
> >>>> +}
> >>>> +
> >>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	percpu_counter_destroy(&lock->writers);
> >>>> +}
> >>>> +
> >>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	if (atomic_read(&lock->readers))
> >>>> +		return false;
> >>>> +
> >>>> +	percpu_counter_inc(&lock->writers);
> >>>> +
> >>>> +	/*
> >>>> +	 * Ensure writers count is updated before we check for
> >>>> +	 * pending readers
> >>>> +	 */
> >>>> +	smp_mb();
> >>>> +	if (atomic_read(&lock->readers)) {
> >>>> +		btrfs_drw_read_unlock(lock);
> >>>> +		return false;
> >>>> +	}
> >>>> +
> >>>> +	return true;
> >>>> +}
> >>>> +
> >>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	while(true) {
> >>>> +		if (btrfs_drw_try_write_lock(lock))
> >>>> +			return;
> >>>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> >>>> +	}
> >>>> +}
> >>>> +
> >>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	percpu_counter_dec(&lock->writers);
> >>>> +	cond_wake_up(&lock->pending_readers);
> >>>> +}
> >>>> +
> >>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	atomic_inc(&lock->readers);
> >>>> +	smp_mb__after_atomic();
> >>>> +
> >>>> +	wait_event(lock->pending_readers,
> >>>> +		   percpu_counter_sum(&lock->writers) == 0);
> >>>> +}
> >>>> +
> >>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> >>>> +{
> >>>> +	/*
> >>>> +	 * Atomic RMW operations imply full barrier, so woken up writers
> >>>> +	 * are guaranteed to see the decrement
> >>>> +	 */
> >>>> +	if (atomic_dec_and_test(&lock->readers))
> >>>> +		wake_up(&lock->pending_writers);
> >>>> +}
> >>>> +
> >>>> +
> >>>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> >>>> new file mode 100644
> >>>> index 000000000000..baff59561c06
> >>>> --- /dev/null
> >>>> +++ b/fs/btrfs/drw_lock.h
> >>>> @@ -0,0 +1,23 @@
> >>>> +#ifndef BTRFS_DRW_LOCK_H
> >>>> +#define BTRFS_DRW_LOCK_H
> >>>> +
> >>>> +#include <linux/atomic.h>
> >>>> +#include <linux/wait.h>
> >>>> +#include <linux/percpu_counter.h>
> >>>> +
> >>>> +struct btrfs_drw_lock {
> >>>> +	atomic_t readers;
> >>>> +	struct percpu_counter writers;
> >>>> +	wait_queue_head_t pending_writers;
> >>>> +	wait_queue_head_t pending_readers;
> >>>> +};
> >>>> +
> >>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> >>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> >>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> >>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> >>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> >>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> >>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> >>>> +
> >>>> +#endif
> >>>> -- 
> >>>> 2.17.1
> >>>>
> >>>
> >>>
> >>
> > 
> > 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

On 8.06.19 г. 19:06 ч., Paul E. McKenney wrote:
> On Sat, Jun 08, 2019 at 06:44:17PM +0300, Nikolay Borisov wrote:
>> On 8.06.19 г. 18:13 ч., Paul E. McKenney wrote:
>>> On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
>>>> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
>>>>> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
>>>>>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
>>>>>> to have multiple readers or multiple writers but not multiple readers
>>>>>> and writers holding it concurrently. The code is factored out from
>>>>>> the existing open-coded locking scheme used to exclude pending
>>>>>> snapshots from nocow writers and vice-versa. Current implementation
>>>>>> actually favors Readers (that is snapshot creaters) to writers (nocow
>>>>>> writers of the filesystem).
>>>>>>
>>>>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>>>>>
>>>>> A preliminary question...
>>>>>
>>>>> What prevents the following sequence of events from happening?
>>>>>
>>>>> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
>>>>> 	which sees that lock->readers is zero and thus executes
>>>>> 	percpu_counter_inc(&lock->writers).
>>>>>
>>>>> o	btrfs_drw_read_lock() increments lock->readers, does the
>>>>> 	smp_mb__after_atomic(), and then does the wait_event().
>>>>> 	Because btrfs_drw_try_write_lock() incremented its CPU's
>>>>> 	lock->writers, the sum is the value one, so it blocks.
>>>>>
>>>>> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
>>>>> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
>>>>> 	(which decrements the current CPU's counter, so that a future
>>>>> 	sum would get zero), and returns false.
>>>>
>>>> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
>>>> already pointed that out and I've fixed it.
>>>
>>> Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?
> 
> Right...  Make that kernel/locking/locktorture.c
> 
>> At the moment - nothing. I rely on the fact that the original code I
>> extracted that from is bug-free (ha-ha). So perhahps hooking up
>> locktorture seems like a good suggestion. From a quick look I guess I
>> could mostly model that lock against the rwsem. The question is how do I
>> model the trylock semantics as well as the "double" part?
> 
> Implementing a correct synchronization primitive is like committing the
> perfect crime.  There are at least 50 things that can go wrong, and if
> you are a highly experienced genius, you -might- be able to anticipate
> and handle 25 of them.  (With apologies to any Kathleen Turner fans who
> might still be alive.)  Please note that this still applies to code
> ported from somewhere else because different environments likely have
> different assumptions and properties.

I agree, I'm far from thinking that the locking scheme is actually bug
free (hence the 'ha-ha') I'm not that arrogant (yet).

> 
> Therefore, heavy-duty stress testing is not optional.  In fact, formal
> verification is becoming non-optional as well -- please see Catalin
> Marinas's work on verifying the Linux kernel's queued spinlock for
> an example.

I assume you are referring to "Formal Methods for kernel hackers"? If
so, TLA+ has been on my radar ever since
https://lamport.azurewebsites.net/tla/formal-methods-amazon.pdf .

However I've yet to invest the time to be able to properly model a real
protocol (be it locking or otherwise) in it. Perhahps I could use the
DRW lock as a learning opportunity, we'll see.

> 
> You are right, current locktorture would get upset about having concurrent
> writers.  To teach locktorture about this, I suggest adding a flag to
> the lock_torture_ops structure named something like concurrent_write,
> but hopefully shorter.  Then this flag can be used to disable the "only
> one writer" check in lock_torture_writer().
> 
> Seem reasonable?

Good idea, I'll see to extending lock-torture but this will happen in a
week or so because I'm about to go on a holiday.

> 
> 							Thanx, Paul
> 
>>>> The idea here is that if a reader came after we've incremented out
>>>> percpu counter then it would have blocked, the writer would see that and
>>>> invoke btrfs_drw_write_unlock which will decrement the percpu counter
>>>> and will wakeup the reader that is now blocked on pending_readers.
>>>
>>> OK, I will await your next version.
>>>
>>> 							Thanx, Paul
>>>
>>>>> o	btrfs_drw_write_lock() therefore does its wait_event().
>>>>> 	Because lock->readers is nonzero, it blocks.
>>>>>
>>>>> o	Both tasks are now blocked.  In the absence of future calls
>>>>> 	to these functions (and perhaps even given such future calls),
>>>>> 	we have deadlock.
>>>>>
>>>>> So what am I missing here?
>>>>>
>>>>> 							Thanx, Paul
>>>>>
>>>>>> ---
>>>>>>  fs/btrfs/Makefile   |  2 +-
>>>>>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>>>>>>  3 files changed, 95 insertions(+), 1 deletion(-)
>>>>>>  create mode 100644 fs/btrfs/drw_lock.c
>>>>>>  create mode 100644 fs/btrfs/drw_lock.h
>>>>>>
>>>>>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
>>>>>> index ca693dd554e9..dc60127791e6 100644
>>>>>> --- a/fs/btrfs/Makefile
>>>>>> +++ b/fs/btrfs/Makefile
>>>>>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>>>>>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>>>>>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>>>>>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
>>>>>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
>>>>>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>>>>>>  
>>>>>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>>>>>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
>>>>>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
>>>>>> new file mode 100644
>>>>>> index 000000000000..9681bf7544be
>>>>>> --- /dev/null
>>>>>> +++ b/fs/btrfs/drw_lock.c
>>>>>> @@ -0,0 +1,71 @@
>>>>>> +#include "drw_lock.h"
>>>>>> +#include "ctree.h"
>>>>>> +
>>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	atomic_set(&lock->readers, 0);
>>>>>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
>>>>>> +	init_waitqueue_head(&lock->pending_readers);
>>>>>> +	init_waitqueue_head(&lock->pending_writers);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	percpu_counter_destroy(&lock->writers);
>>>>>> +}
>>>>>> +
>>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	if (atomic_read(&lock->readers))
>>>>>> +		return false;
>>>>>> +
>>>>>> +	percpu_counter_inc(&lock->writers);
>>>>>> +
>>>>>> +	/*
>>>>>> +	 * Ensure writers count is updated before we check for
>>>>>> +	 * pending readers
>>>>>> +	 */
>>>>>> +	smp_mb();
>>>>>> +	if (atomic_read(&lock->readers)) {
>>>>>> +		btrfs_drw_read_unlock(lock);
>>>>>> +		return false;
>>>>>> +	}
>>>>>> +
>>>>>> +	return true;
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	while(true) {
>>>>>> +		if (btrfs_drw_try_write_lock(lock))
>>>>>> +			return;
>>>>>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
>>>>>> +	}
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	percpu_counter_dec(&lock->writers);
>>>>>> +	cond_wake_up(&lock->pending_readers);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	atomic_inc(&lock->readers);
>>>>>> +	smp_mb__after_atomic();
>>>>>> +
>>>>>> +	wait_event(lock->pending_readers,
>>>>>> +		   percpu_counter_sum(&lock->writers) == 0);
>>>>>> +}
>>>>>> +
>>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
>>>>>> +{
>>>>>> +	/*
>>>>>> +	 * Atomic RMW operations imply full barrier, so woken up writers
>>>>>> +	 * are guaranteed to see the decrement
>>>>>> +	 */
>>>>>> +	if (atomic_dec_and_test(&lock->readers))
>>>>>> +		wake_up(&lock->pending_writers);
>>>>>> +}
>>>>>> +
>>>>>> +
>>>>>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
>>>>>> new file mode 100644
>>>>>> index 000000000000..baff59561c06
>>>>>> --- /dev/null
>>>>>> +++ b/fs/btrfs/drw_lock.h
>>>>>> @@ -0,0 +1,23 @@
>>>>>> +#ifndef BTRFS_DRW_LOCK_H
>>>>>> +#define BTRFS_DRW_LOCK_H
>>>>>> +
>>>>>> +#include <linux/atomic.h>
>>>>>> +#include <linux/wait.h>
>>>>>> +#include <linux/percpu_counter.h>
>>>>>> +
>>>>>> +struct btrfs_drw_lock {
>>>>>> +	atomic_t readers;
>>>>>> +	struct percpu_counter writers;
>>>>>> +	wait_queue_head_t pending_writers;
>>>>>> +	wait_queue_head_t pending_readers;
>>>>>> +};
>>>>>> +
>>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
>>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
>>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
>>>>>> +
>>>>>> +#endif
>>>>>> -- 
>>>>>> 2.17.1
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>
> 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Andrea Parri]

On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Interesting!  Thank you for sending this over, Nikolay.

I only have a couple of nits (below) to add:

[...]

> +
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> +{
> +	/*
> +	 * Atomic RMW operations imply full barrier, so woken up writers
> +	 * are guaranteed to see the decrement
> +	 */

Not every atomic RMW operations imply a full barrier (as exemplified,
e.g., by the atomic_inc() in btrfs_drw_read_lock()); maybe simply

  s/Atomic RMW operations imply/atomic_dec_and_test() implies/

FYI, checkpatch.pl issues a few warnings on this patch (you may want
to address some of them in the next version).

Thanks,
  Andrea


> +	if (atomic_dec_and_test(&lock->readers))
> +		wake_up(&lock->pending_writers);
> +}
> +
> +

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Paul E. McKenney]

On Sat, Jun 08, 2019 at 07:21:53PM +0300, Nikolay Borisov wrote:
> On 8.06.19 г. 19:06 ч., Paul E. McKenney wrote:
> > On Sat, Jun 08, 2019 at 06:44:17PM +0300, Nikolay Borisov wrote:
> >> On 8.06.19 г. 18:13 ч., Paul E. McKenney wrote:
> >>> On Fri, Jun 07, 2019 at 02:59:34PM +0300, Nikolay Borisov wrote:
> >>>> On 7.06.19 г. 13:52 ч., Paul E. McKenney wrote:
> >>>>> On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> >>>>>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> >>>>>> to have multiple readers or multiple writers but not multiple readers
> >>>>>> and writers holding it concurrently. The code is factored out from
> >>>>>> the existing open-coded locking scheme used to exclude pending
> >>>>>> snapshots from nocow writers and vice-versa. Current implementation
> >>>>>> actually favors Readers (that is snapshot creaters) to writers (nocow
> >>>>>> writers of the filesystem).
> >>>>>>
> >>>>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> >>>>>
> >>>>> A preliminary question...
> >>>>>
> >>>>> What prevents the following sequence of events from happening?
> >>>>>
> >>>>> o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
> >>>>> 	which sees that lock->readers is zero and thus executes
> >>>>> 	percpu_counter_inc(&lock->writers).
> >>>>>
> >>>>> o	btrfs_drw_read_lock() increments lock->readers, does the
> >>>>> 	smp_mb__after_atomic(), and then does the wait_event().
> >>>>> 	Because btrfs_drw_try_write_lock() incremented its CPU's
> >>>>> 	lock->writers, the sum is the value one, so it blocks.
> >>>>>
> >>>>> o	btrfs_drw_try_write_lock() checks lock->readers, sees that
> >>>>> 	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
> >>>>> 	(which decrements the current CPU's counter, so that a future
> >>>>> 	sum would get zero), and returns false.
> >>>>
> >>>> btrfs_drw_read_unlock is actually btrfs_drw_write_unlock, my bad, Filipe
> >>>> already pointed that out and I've fixed it.
> >>>
> >>> Ah!  I must then ask what you are using to test this.  kernel/locktorture.c?
> > 
> > Right...  Make that kernel/locking/locktorture.c
> > 
> >> At the moment - nothing. I rely on the fact that the original code I
> >> extracted that from is bug-free (ha-ha). So perhahps hooking up
> >> locktorture seems like a good suggestion. From a quick look I guess I
> >> could mostly model that lock against the rwsem. The question is how do I
> >> model the trylock semantics as well as the "double" part?
> > 
> > Implementing a correct synchronization primitive is like committing the
> > perfect crime.  There are at least 50 things that can go wrong, and if
> > you are a highly experienced genius, you -might- be able to anticipate
> > and handle 25 of them.  (With apologies to any Kathleen Turner fans who
> > might still be alive.)  Please note that this still applies to code
> > ported from somewhere else because different environments likely have
> > different assumptions and properties.
> 
> I agree, I'm far from thinking that the locking scheme is actually bug
> free (hence the 'ha-ha') I'm not that arrogant (yet).

;-) ;-) ;-)

> > Therefore, heavy-duty stress testing is not optional.  In fact, formal
> > verification is becoming non-optional as well -- please see Catalin
> > Marinas's work on verifying the Linux kernel's queued spinlock for
> > an example.
> 
> I assume you are referring to "Formal Methods for kernel hackers"? If
> so, TLA+ has been on my radar ever since
> https://lamport.azurewebsites.net/tla/formal-methods-amazon.pdf .

Yes, and good to hear.  There are other options, including Promela/spin,
cbmc, and so on.

> However I've yet to invest the time to be able to properly model a real
> protocol (be it locking or otherwise) in it. Perhahps I could use the
> DRW lock as a learning opportunity, we'll see.

The learning would likely be reusable, and might pay for itself in
terms of bugs found more quickly and with less effort.  Mileage can
vary, as always, of course.

> > You are right, current locktorture would get upset about having concurrent
> > writers.  To teach locktorture about this, I suggest adding a flag to
> > the lock_torture_ops structure named something like concurrent_write,
> > but hopefully shorter.  Then this flag can be used to disable the "only
> > one writer" check in lock_torture_writer().
> > 
> > Seem reasonable?
> 
> Good idea, I'll see to extending lock-torture but this will happen in a
> week or so because I'm about to go on a holiday.

Have a great holiday, and looking forward to seeing your next version
and locktorture modifications.

							Thanx, Paul

> >>>> The idea here is that if a reader came after we've incremented out
> >>>> percpu counter then it would have blocked, the writer would see that and
> >>>> invoke btrfs_drw_write_unlock which will decrement the percpu counter
> >>>> and will wakeup the reader that is now blocked on pending_readers.
> >>>
> >>> OK, I will await your next version.
> >>>
> >>> 							Thanx, Paul
> >>>
> >>>>> o	btrfs_drw_write_lock() therefore does its wait_event().
> >>>>> 	Because lock->readers is nonzero, it blocks.
> >>>>>
> >>>>> o	Both tasks are now blocked.  In the absence of future calls
> >>>>> 	to these functions (and perhaps even given such future calls),
> >>>>> 	we have deadlock.
> >>>>>
> >>>>> So what am I missing here?
> >>>>>
> >>>>> 							Thanx, Paul
> >>>>>
> >>>>>> ---
> >>>>>>  fs/btrfs/Makefile   |  2 +-
> >>>>>>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
> >>>>>>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
> >>>>>>  3 files changed, 95 insertions(+), 1 deletion(-)
> >>>>>>  create mode 100644 fs/btrfs/drw_lock.c
> >>>>>>  create mode 100644 fs/btrfs/drw_lock.h
> >>>>>>
> >>>>>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> >>>>>> index ca693dd554e9..dc60127791e6 100644
> >>>>>> --- a/fs/btrfs/Makefile
> >>>>>> +++ b/fs/btrfs/Makefile
> >>>>>> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
> >>>>>>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
> >>>>>>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
> >>>>>>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> >>>>>> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
> >>>>>> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
> >>>>>>  
> >>>>>>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
> >>>>>>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> >>>>>> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> >>>>>> new file mode 100644
> >>>>>> index 000000000000..9681bf7544be
> >>>>>> --- /dev/null
> >>>>>> +++ b/fs/btrfs/drw_lock.c
> >>>>>> @@ -0,0 +1,71 @@
> >>>>>> +#include "drw_lock.h"
> >>>>>> +#include "ctree.h"
> >>>>>> +
> >>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	atomic_set(&lock->readers, 0);
> >>>>>> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> >>>>>> +	init_waitqueue_head(&lock->pending_readers);
> >>>>>> +	init_waitqueue_head(&lock->pending_writers);
> >>>>>> +}
> >>>>>> +
> >>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	percpu_counter_destroy(&lock->writers);
> >>>>>> +}
> >>>>>> +
> >>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	if (atomic_read(&lock->readers))
> >>>>>> +		return false;
> >>>>>> +
> >>>>>> +	percpu_counter_inc(&lock->writers);
> >>>>>> +
> >>>>>> +	/*
> >>>>>> +	 * Ensure writers count is updated before we check for
> >>>>>> +	 * pending readers
> >>>>>> +	 */
> >>>>>> +	smp_mb();
> >>>>>> +	if (atomic_read(&lock->readers)) {
> >>>>>> +		btrfs_drw_read_unlock(lock);
> >>>>>> +		return false;
> >>>>>> +	}
> >>>>>> +
> >>>>>> +	return true;
> >>>>>> +}
> >>>>>> +
> >>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	while(true) {
> >>>>>> +		if (btrfs_drw_try_write_lock(lock))
> >>>>>> +			return;
> >>>>>> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> >>>>>> +	}
> >>>>>> +}
> >>>>>> +
> >>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	percpu_counter_dec(&lock->writers);
> >>>>>> +	cond_wake_up(&lock->pending_readers);
> >>>>>> +}
> >>>>>> +
> >>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	atomic_inc(&lock->readers);
> >>>>>> +	smp_mb__after_atomic();
> >>>>>> +
> >>>>>> +	wait_event(lock->pending_readers,
> >>>>>> +		   percpu_counter_sum(&lock->writers) == 0);
> >>>>>> +}
> >>>>>> +
> >>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> >>>>>> +{
> >>>>>> +	/*
> >>>>>> +	 * Atomic RMW operations imply full barrier, so woken up writers
> >>>>>> +	 * are guaranteed to see the decrement
> >>>>>> +	 */
> >>>>>> +	if (atomic_dec_and_test(&lock->readers))
> >>>>>> +		wake_up(&lock->pending_writers);
> >>>>>> +}
> >>>>>> +
> >>>>>> +
> >>>>>> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> >>>>>> new file mode 100644
> >>>>>> index 000000000000..baff59561c06
> >>>>>> --- /dev/null
> >>>>>> +++ b/fs/btrfs/drw_lock.h
> >>>>>> @@ -0,0 +1,23 @@
> >>>>>> +#ifndef BTRFS_DRW_LOCK_H
> >>>>>> +#define BTRFS_DRW_LOCK_H
> >>>>>> +
> >>>>>> +#include <linux/atomic.h>
> >>>>>> +#include <linux/wait.h>
> >>>>>> +#include <linux/percpu_counter.h>
> >>>>>> +
> >>>>>> +struct btrfs_drw_lock {
> >>>>>> +	atomic_t readers;
> >>>>>> +	struct percpu_counter writers;
> >>>>>> +	wait_queue_head_t pending_writers;
> >>>>>> +	wait_queue_head_t pending_readers;
> >>>>>> +};
> >>>>>> +
> >>>>>> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> >>>>>> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> >>>>>> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> >>>>>> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> >>>>>> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> >>>>>> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> >>>>>> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> >>>>>> +
> >>>>>> +#endif
> >>>>>> -- 
> >>>>>> 2.17.1
> >>>>>>
> >>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >>
> > 
> > 
> 

Re: [PATCH 1/2] btrfs: Implement DRW lock

[David Sterba]

On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/Makefile   |  2 +-
>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/drw_lock.h | 23 +++++++++++++++

In v2, please use locking.[ch] instead of new files.

[PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
to have multiple readers or multiple writers but not multiple readers
and writers holding it concurrently. The code is factored out from
the existing open-coded locking scheme used to exclude pending
snapshots from nocow writers and vice-versa. Current implementation
actually favors Readers (that is snapshot creaters) to writers (nocow
writers of the filesystem).

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/ctree.h   |  1 +
 fs/btrfs/locking.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/locking.h | 21 +++++++++++
 3 files changed, 112 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index ad275d06e95f..efc2cd147141 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -33,6 +33,7 @@
 #include "extent_map.h"
 #include "async-thread.h"
 #include "block-rsv.h"
+#include "locking.h"
 
 struct btrfs_trans_handle;
 struct btrfs_transaction;
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index e713900f96b6..d890833694c9 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -565,3 +565,93 @@ struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
 	}
 	return eb;
 }
+
+/*
+ * DRW stands for double-reader-writer lock. It's used in situation where you
+ * want to provide A-B exclusion but not AA or BB. Currently implementation
+ * gives more priority to reader. If a reader and a writer both race to acquire
+ * their respective sides of the lock the writer would yield its lock as soon as
+ * it detects a concurrent reader. Additionally if there are pending readers no
+ * new writers would be allowed to come in and acquire the lock.
+ */
+int btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
+{
+	int ret;
+
+	ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
+	if (ret)
+		return ret;
+
+	atomic_set(&lock->readers, 0);
+	init_waitqueue_head(&lock->pending_readers);
+	init_waitqueue_head(&lock->pending_writers);
+
+	return 0;
+}
+
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_destroy(&lock->writers);
+}
+
+/* Return true if acquisition is successful, false otherwise */
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
+{
+	if (atomic_read(&lock->readers))
+		return false;
+
+	percpu_counter_inc(&lock->writers);
+
+	/*
+	 * Ensure writers count is updated before we check for
+	 * pending readers
+	 */
+	smp_mb();
+	if (atomic_read(&lock->readers)) {
+		btrfs_drw_write_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
+{
+	while (true) {
+		if (btrfs_drw_try_write_lock(lock))
+			return;
+		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
+	}
+}
+
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_dec(&lock->writers);
+	cond_wake_up(&lock->pending_readers);
+}
+
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
+{
+	atomic_inc(&lock->readers);
+
+	/*
+	 * Ensure the pending reader count is perceieved BEFORE this reader
+	 * goes to sleep in case of active writers. This guarantees new writers
+	 * won't be allowed and that the current reader will be woken up when
+	 * the last active writer finishes its jobs.
+	 */
+	smp_mb__after_atomic();
+
+	wait_event(lock->pending_readers,
+		   percpu_counter_sum(&lock->writers) == 0);
+}
+
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
+{
+	/*
+	 * atomic_dec_and_test implies a full barrier, so woken up writers
+	 * are guaranteed to see the decrement
+	 */
+	if (atomic_dec_and_test(&lock->readers))
+		wake_up(&lock->pending_writers);
+}
diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index 21a285883e89..ba60318c53d5 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -7,12 +7,17 @@
 #define BTRFS_LOCKING_H
 
 #include "extent_io.h"
+#include <linux/atomic.h>
+#include <linux/wait.h>
+#include <linux/percpu_counter.h>
 
 #define BTRFS_WRITE_LOCK 1
 #define BTRFS_READ_LOCK 2
 #define BTRFS_WRITE_LOCK_BLOCKING 3
 #define BTRFS_READ_LOCK_BLOCKING 4
 
+struct btrfs_path;
+
 void btrfs_tree_lock(struct extent_buffer *eb);
 void btrfs_tree_unlock(struct extent_buffer *eb);
 
@@ -48,4 +53,20 @@ static inline void btrfs_tree_unlock_rw(struct extent_buffer *eb, int rw)
 		BUG();
 }
 
+
+struct btrfs_drw_lock {
+	atomic_t readers;
+	struct percpu_counter writers;
+	wait_queue_head_t pending_writers;
+	wait_queue_head_t pending_readers;
+};
+
+int btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
+
 #endif
-- 
2.17.1

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Christoph Hellwig]

On Thu, Jan 30, 2020 at 03:42:27PM +0200, Nikolay Borisov wrote:
> 
> 
> On 30.01.20 г. 15:41 ч., Christoph Hellwig wrote:
> > On Thu, Jan 30, 2020 at 02:59:41PM +0200, Nikolay Borisov wrote:
> >> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> >> to have multiple readers or multiple writers but not multiple readers
> >> and writers holding it concurrently. The code is factored out from
> >> the existing open-coded locking scheme used to exclude pending
> >> snapshots from nocow writers and vice-versa. Current implementation
> >> actually favors Readers (that is snapshot creaters) to writers (nocow
> >> writers of the filesystem).
> > 
> > Any reason not to move it to lib/ under a new option so that other
> > users could reuse it as needed?
> > 
> 
> Yes, I think it's rather specific to btrfs. I don't care honestly if
> there is demand I wouldn't mind.

Ok, just keep it in btrfs for now, we can always move it later.
It just looks like very generic code.

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

On 30.01.20 г. 15:41 ч., Christoph Hellwig wrote:
> On Thu, Jan 30, 2020 at 02:59:41PM +0200, Nikolay Borisov wrote:
>> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
>> to have multiple readers or multiple writers but not multiple readers
>> and writers holding it concurrently. The code is factored out from
>> the existing open-coded locking scheme used to exclude pending
>> snapshots from nocow writers and vice-versa. Current implementation
>> actually favors Readers (that is snapshot creaters) to writers (nocow
>> writers of the filesystem).
> 
> Any reason not to move it to lib/ under a new option so that other
> users could reuse it as needed?
> 

Yes, I think it's rather specific to btrfs. I don't care honestly if
there is demand I wouldn't mind.

Re: [PATCH 1/2] btrfs: Implement DRW lock

[Christoph Hellwig]

On Thu, Jan 30, 2020 at 02:59:41PM +0200, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).

Any reason not to move it to lib/ under a new option so that other
users could reuse it as needed?

[PATCH 1/2] btrfs: Implement DRW lock

[Nikolay Borisov]

A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
to have multiple readers or multiple writers but not multiple readers
and writers holding it concurrently. The code is factored out from
the existing open-coded locking scheme used to exclude pending
snapshots from nocow writers and vice-versa. Current implementation
actually favors Readers (that is snapshot creaters) to writers (nocow
writers of the filesystem).

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/Makefile   |  2 +-
 fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/drw_lock.h | 23 +++++++++++++++
 3 files changed, 95 insertions(+), 1 deletion(-)
 create mode 100644 fs/btrfs/drw_lock.c
 create mode 100644 fs/btrfs/drw_lock.h

diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index ca693dd554e9..dc60127791e6 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
 	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
 	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
-	   uuid-tree.o props.o free-space-tree.o tree-checker.o
+	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
 
 btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
 btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
new file mode 100644
index 000000000000..9681bf7544be
--- /dev/null
+++ b/fs/btrfs/drw_lock.c
@@ -0,0 +1,71 @@
+#include "drw_lock.h"
+#include "ctree.h"
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
+{
+	atomic_set(&lock->readers, 0);
+	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
+	init_waitqueue_head(&lock->pending_readers);
+	init_waitqueue_head(&lock->pending_writers);
+}
+
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_destroy(&lock->writers);
+}
+
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
+{
+	if (atomic_read(&lock->readers))
+		return false;
+
+	percpu_counter_inc(&lock->writers);
+
+	/*
+	 * Ensure writers count is updated before we check for
+	 * pending readers
+	 */
+	smp_mb();
+	if (atomic_read(&lock->readers)) {
+		btrfs_drw_read_unlock(lock);
+		return false;
+	}
+
+	return true;
+}
+
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
+{
+	while(true) {
+		if (btrfs_drw_try_write_lock(lock))
+			return;
+		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
+	}
+}
+
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
+{
+	percpu_counter_dec(&lock->writers);
+	cond_wake_up(&lock->pending_readers);
+}
+
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
+{
+	atomic_inc(&lock->readers);
+	smp_mb__after_atomic();
+
+	wait_event(lock->pending_readers,
+		   percpu_counter_sum(&lock->writers) == 0);
+}
+
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
+{
+	/*
+	 * Atomic RMW operations imply full barrier, so woken up writers
+	 * are guaranteed to see the decrement
+	 */
+	if (atomic_dec_and_test(&lock->readers))
+		wake_up(&lock->pending_writers);
+}
+
+
diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
new file mode 100644
index 000000000000..baff59561c06
--- /dev/null
+++ b/fs/btrfs/drw_lock.h
@@ -0,0 +1,23 @@
+#ifndef BTRFS_DRW_LOCK_H
+#define BTRFS_DRW_LOCK_H
+
+#include <linux/atomic.h>
+#include <linux/wait.h>
+#include <linux/percpu_counter.h>
+
+struct btrfs_drw_lock {
+	atomic_t readers;
+	struct percpu_counter writers;
+	wait_queue_head_t pending_writers;
+	wait_queue_head_t pending_readers;
+};
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
+
+#endif
-- 
2.17.1