* [PATCH 1/2] btrfs: Simplify snapshot exclusion code
@ 2019-04-23 11:42 Nikolay Borisov
2019-04-23 11:42 ` [PATCH 2/2] btrfs: Remove dead code Nikolay Borisov
2019-04-24 9:49 ` [PATCH 1/2] btrfs: Simplify snapshot exclusion code Filipe Manana
0 siblings, 2 replies; 6+ messages in thread
From: Nikolay Borisov @ 2019-04-23 11:42 UTC (permalink / raw)
To: linux-btrfs; +Cc: Nikolay Borisov
BTRFS sports a mechanism to provide exclusion when a snapshot is about
to be created. This is implemented via btrfs_start_write_no_snapshotting
et al. Currently the implementation of that mechanism is some perverse
amalgamation of a percpu variable, an explicit waitqueue, an atomic_t
variable and an implicit wait bit on said atomic_t via wait_var_event
family of API. And for good measure there is a memory barrier thrown in
the mix...
Astute reader should have concluded by now that it's bordering on
impossible to prove whether this scheme works. What's worse - all of
this is required to achieve something really simple - ensure certain
operations cannot run during snapshot creation. Let's simplify this by
relying on a single atomic_t used as a boolean flag. This commit changes
only the implementation and not the semantics of the existing mechanism.
Now, if the atomic is 1 (snapshot is in progress) callers of
btrfs_start_write_no_snapshotting will get a ret val of 0 that should be
handled accordingly.
btrfs_wait_for_snapshot_creation OTOH will block until snapshotting is
in progress and return when current snapshot in progress is finished and
will acquire the right to create a snapshot.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
fs/btrfs/extent-tree.c | 20 +++++---------------
fs/btrfs/ioctl.c | 9 ++-------
2 files changed, 7 insertions(+), 22 deletions(-)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 8f2b7b29c3fd..d9e2e35700fd 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -11333,25 +11333,15 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
*/
void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
{
- percpu_counter_dec(&root->subv_writers->counter);
- cond_wake_up(&root->subv_writers->wait);
+ ASSERT(atomic_read(&root->will_be_snapshotted) == 1);
+ if (atomic_dec_and_test(&root->will_be_snapshotted))
+ wake_up_var(&root->will_be_snapshotted);
}
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;
+ ASSERT(atomic_read(&root->will_be_snapshotted) >= 0);
+ return atomic_add_unless(&root->will_be_snapshotted, 1, 1);
}
void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 8774d4be7c97..f9f66c8a5dad 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -794,11 +794,7 @@ 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_wait_for_snapshot_creation(root);
ret = btrfs_start_delalloc_snapshot(root);
if (ret)
@@ -878,8 +874,7 @@ 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_end_write_no_snapshotting(root);
free_pending:
kfree(pending_snapshot->root_item);
btrfs_free_path(pending_snapshot->path);
--
2.17.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH 2/2] btrfs: Remove dead code
2019-04-23 11:42 [PATCH 1/2] btrfs: Simplify snapshot exclusion code Nikolay Borisov
@ 2019-04-23 11:42 ` Nikolay Borisov
2019-04-23 13:58 ` [PATCH v2] " Nikolay Borisov
2019-04-24 9:49 ` [PATCH 1/2] btrfs: Simplify snapshot exclusion code Filipe Manana
1 sibling, 1 reply; 6+ messages in thread
From: Nikolay Borisov @ 2019-04-23 11:42 UTC (permalink / raw)
To: linux-btrfs; +Cc: Nikolay Borisov
BTRFS no longer relies on btrfs_subvolume_writers for snapshot
exclusion. Just remove any code allocating/freeing it and the structure
definition itself.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
fs/btrfs/ctree.h | 6 ------
fs/btrfs/disk-io.c | 10 ----------
2 files changed, 16 deletions(-)
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 05731e4ca358..9fdb7ab74102 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1169,11 +1169,6 @@ 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
*/
@@ -1339,7 +1334,6 @@ struct btrfs_root {
* manipulation with the read-only status via SUBVOL_SETFLAGS
*/
int send_in_progress;
- struct btrfs_subvolume_writers *subv_writers;
atomic_t will_be_snapshotted;
atomic_t snapshot_force_cow;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 005c9f5c6f10..ad2fa12cc654 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1492,7 +1492,6 @@ 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),
@@ -1502,13 +1501,6 @@ 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_init_free_ino_ctl(root);
spin_lock_init(&root->ino_cache_lock);
init_waitqueue_head(&root->ino_cache_wait);
@@ -3870,8 +3862,6 @@ 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);
free_extent_buffer(root->node);
free_extent_buffer(root->commit_root);
kfree(root->free_ino_ctl);
--
2.17.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH v2] btrfs: Remove dead code
2019-04-23 11:42 ` [PATCH 2/2] btrfs: Remove dead code Nikolay Borisov
@ 2019-04-23 13:58 ` Nikolay Borisov
2019-04-23 14:49 ` [PATCH v3] " Nikolay Borisov
0 siblings, 1 reply; 6+ messages in thread
From: Nikolay Borisov @ 2019-04-23 13:58 UTC (permalink / raw)
To: linux-btrfs; +Cc: Nikolay Borisov
BTRFS no longer relies on btrfs_subvolume_writers for snapshot
exclusion. Just remove any code allocating/freeing it and the structure
definition itself.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
Changes in v2:
* Remove definition of btrfs_alloc_subvolume_writers.
fs/btrfs/ctree.h | 6 ------
fs/btrfs/disk-io.c | 29 -----------------------------
2 files changed, 35 deletions(-)
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 05731e4ca358..9fdb7ab74102 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1169,11 +1169,6 @@ 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
*/
@@ -1339,7 +1334,6 @@ struct btrfs_root {
* manipulation with the read-only status via SUBVOL_SETFLAGS
*/
int send_in_progress;
- struct btrfs_subvolume_writers *subv_writers;
atomic_t will_be_snapshotted;
atomic_t snapshot_force_cow;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 005c9f5c6f10..411678c88047 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1130,25 +1130,6 @@ 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)
{
@@ -1492,7 +1473,6 @@ 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),
@@ -1502,13 +1482,6 @@ 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_init_free_ino_ctl(root);
spin_lock_init(&root->ino_cache_lock);
init_waitqueue_head(&root->ino_cache_wait);
@@ -3870,8 +3843,6 @@ 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);
free_extent_buffer(root->node);
free_extent_buffer(root->commit_root);
kfree(root->free_ino_ctl);
--
2.17.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH v3] btrfs: Remove dead code
2019-04-23 13:58 ` [PATCH v2] " Nikolay Borisov
@ 2019-04-23 14:49 ` Nikolay Borisov
0 siblings, 0 replies; 6+ messages in thread
From: Nikolay Borisov @ 2019-04-23 14:49 UTC (permalink / raw)
To: linux-btrfs; +Cc: Nikolay Borisov
BTRFS no longer relies on btrfs_subvolume_writers for snapshot
exclusion. Just remove any code allocating/freeing it and the structure
definition itself.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
Changes in v3:
* Removed -btrfs_free_subvolume_writers. This is really the final piece of the
puzzle....
Changes in v2:
* Remove definition of btrfs_alloc_subvolume_writers.
fs/btrfs/ctree.h | 6 ------
fs/btrfs/disk-io.c | 36 ------------------------------------
2 files changed, 42 deletions(-)
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 05731e4ca358..9fdb7ab74102 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1169,11 +1169,6 @@ 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
*/
@@ -1339,7 +1334,6 @@ struct btrfs_root {
* manipulation with the read-only status via SUBVOL_SETFLAGS
*/
int send_in_progress;
- struct btrfs_subvolume_writers *subv_writers;
atomic_t will_be_snapshotted;
atomic_t snapshot_force_cow;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 005c9f5c6f10..4702227d9ddd 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1130,32 +1130,6 @@ 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)
{
@@ -1492,7 +1466,6 @@ 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),
@@ -1502,13 +1475,6 @@ 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_init_free_ino_ctl(root);
spin_lock_init(&root->ino_cache_lock);
init_waitqueue_head(&root->ino_cache_wait);
@@ -3870,8 +3836,6 @@ 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);
free_extent_buffer(root->node);
free_extent_buffer(root->commit_root);
kfree(root->free_ino_ctl);
--
2.17.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH 1/2] btrfs: Simplify snapshot exclusion code
2019-04-23 11:42 [PATCH 1/2] btrfs: Simplify snapshot exclusion code Nikolay Borisov
2019-04-23 11:42 ` [PATCH 2/2] btrfs: Remove dead code Nikolay Borisov
@ 2019-04-24 9:49 ` Filipe Manana
2019-04-24 10:19 ` Filipe Manana
1 sibling, 1 reply; 6+ messages in thread
From: Filipe Manana @ 2019-04-24 9:49 UTC (permalink / raw)
To: Nikolay Borisov; +Cc: linux-btrfs
On Tue, Apr 23, 2019 at 12:43 PM Nikolay Borisov <nborisov@suse.com> wrote:
>
> BTRFS sports a mechanism to provide exclusion when a snapshot is about
> to be created. This is implemented via btrfs_start_write_no_snapshotting
> et al. Currently the implementation of that mechanism is some perverse
> amalgamation of a percpu variable, an explicit waitqueue, an atomic_t
> variable and an implicit wait bit on said atomic_t via wait_var_event
> family of API. And for good measure there is a memory barrier thrown in
> the mix...
>
> Astute reader should have concluded by now that it's bordering on
> impossible to prove whether this scheme works. What's worse - all of
> this is required to achieve something really simple - ensure certain
> operations cannot run during snapshot creation. Let's simplify this by
> relying on a single atomic_t used as a boolean flag.
Nop, can't work as a boolean, see below.
> This commit changes
> only the implementation and not the semantics of the existing mechanism.
>
> Now, if the atomic is 1 (snapshot is in progress) callers of
> btrfs_start_write_no_snapshotting will get a ret val of 0 that should be
> handled accordingly.
>
> btrfs_wait_for_snapshot_creation OTOH will block until snapshotting is
> in progress and return when current snapshot in progress is finished and
> will acquire the right to create a snapshot.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
> fs/btrfs/extent-tree.c | 20 +++++---------------
> fs/btrfs/ioctl.c | 9 ++-------
> 2 files changed, 7 insertions(+), 22 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 8f2b7b29c3fd..d9e2e35700fd 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -11333,25 +11333,15 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
> */
> void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
> {
> - percpu_counter_dec(&root->subv_writers->counter);
> - cond_wake_up(&root->subv_writers->wait);
> + ASSERT(atomic_read(&root->will_be_snapshotted) == 1);
> + if (atomic_dec_and_test(&root->will_be_snapshotted))
> + wake_up_var(&root->will_be_snapshotted);
> }
>
> 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;
> + ASSERT(atomic_read(&root->will_be_snapshotted) >= 0);
> + return atomic_add_unless(&root->will_be_snapshotted, 1, 1);
> }
So if two writes call btrfs_start_write_no_snapshotting(), we end up
with root->will_be_snapshotted == 1.
One task calls btrfs_end_write_no_snapshotting(), it decrements it to
1 - we wake up the snapshot creation task while there's still one
nodatacow writer - this is incorrect.
Now the second task calls btrfs_end_write_no_snapshotting(), sees
root->will_be_snapshotted == 0, assertion failure.
>
> void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 8774d4be7c97..f9f66c8a5dad 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -794,11 +794,7 @@ 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_wait_for_snapshot_creation(root);
This naming is also confusing now. The task that creates a snapshot is
calling btrfs_wait_for_snapshot_creation(), waiting for itself?
>
> ret = btrfs_start_delalloc_snapshot(root);
> if (ret)
> @@ -878,8 +874,7 @@ 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_end_write_no_snapshotting(root);
Also confusing. We are not ending a write operation, we are ending
snapshot creation.
Thanks.
> 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.”
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH 1/2] btrfs: Simplify snapshot exclusion code
2019-04-24 9:49 ` [PATCH 1/2] btrfs: Simplify snapshot exclusion code Filipe Manana
@ 2019-04-24 10:19 ` Filipe Manana
0 siblings, 0 replies; 6+ messages in thread
From: Filipe Manana @ 2019-04-24 10:19 UTC (permalink / raw)
To: Nikolay Borisov; +Cc: linux-btrfs
On Wed, Apr 24, 2019 at 10:49 AM Filipe Manana <fdmanana@gmail.com> wrote:
>
> On Tue, Apr 23, 2019 at 12:43 PM Nikolay Borisov <nborisov@suse.com> wrote:
> >
> > BTRFS sports a mechanism to provide exclusion when a snapshot is about
> > to be created. This is implemented via btrfs_start_write_no_snapshotting
> > et al. Currently the implementation of that mechanism is some perverse
> > amalgamation of a percpu variable, an explicit waitqueue, an atomic_t
> > variable and an implicit wait bit on said atomic_t via wait_var_event
> > family of API. And for good measure there is a memory barrier thrown in
> > the mix...
> >
> > Astute reader should have concluded by now that it's bordering on
> > impossible to prove whether this scheme works. What's worse - all of
> > this is required to achieve something really simple - ensure certain
> > operations cannot run during snapshot creation. Let's simplify this by
> > relying on a single atomic_t used as a boolean flag.
>
> Nop, can't work as a boolean, see below.
>
> > This commit changes
> > only the implementation and not the semantics of the existing mechanism.
> >
> > Now, if the atomic is 1 (snapshot is in progress) callers of
> > btrfs_start_write_no_snapshotting will get a ret val of 0 that should be
> > handled accordingly.
> >
> > btrfs_wait_for_snapshot_creation OTOH will block until snapshotting is
> > in progress and return when current snapshot in progress is finished and
> > will acquire the right to create a snapshot.
> >
> > Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> > ---
> > fs/btrfs/extent-tree.c | 20 +++++---------------
> > fs/btrfs/ioctl.c | 9 ++-------
> > 2 files changed, 7 insertions(+), 22 deletions(-)
> >
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index 8f2b7b29c3fd..d9e2e35700fd 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -11333,25 +11333,15 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
> > */
> > void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
> > {
> > - percpu_counter_dec(&root->subv_writers->counter);
> > - cond_wake_up(&root->subv_writers->wait);
> > + ASSERT(atomic_read(&root->will_be_snapshotted) == 1);
> > + if (atomic_dec_and_test(&root->will_be_snapshotted))
> > + wake_up_var(&root->will_be_snapshotted);
> > }
> >
> > 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;
> > + ASSERT(atomic_read(&root->will_be_snapshotted) >= 0);
> > + return atomic_add_unless(&root->will_be_snapshotted, 1, 1);
> > }
>
> So if two writes call btrfs_start_write_no_snapshotting(), we end up
> with root->will_be_snapshotted == 1.
>
> One task calls btrfs_end_write_no_snapshotting(), it decrements it to
> 1 - we wake up the snapshot creation task while there's still one
> nodatacow writer - this is incorrect.
> Now the second task calls btrfs_end_write_no_snapshotting(), sees
> root->will_be_snapshotted == 0, assertion failure.
Actually take that out, I ignored the return value of atomic_add_unless().
So this change does not allow for concurrent no snapshot writers anymore,
multiple tasks calling btrfs_start_write_no_snapshotting(), only one
succeeds and all the others fail,
so that's a regression from what we currently have.
The rest of the confusing names still applies.
>
> >
> > void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
> > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > index 8774d4be7c97..f9f66c8a5dad 100644
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -794,11 +794,7 @@ 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_wait_for_snapshot_creation(root);
>
> This naming is also confusing now. The task that creates a snapshot is
> calling btrfs_wait_for_snapshot_creation(), waiting for itself?
>
> >
> > ret = btrfs_start_delalloc_snapshot(root);
> > if (ret)
> > @@ -878,8 +874,7 @@ 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_end_write_no_snapshotting(root);
>
> Also confusing. We are not ending a write operation, we are ending
> snapshot creation.
>
> Thanks.
>
> > 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.”
--
Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2019-04-24 10:19 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-04-23 11:42 [PATCH 1/2] btrfs: Simplify snapshot exclusion code Nikolay Borisov
2019-04-23 11:42 ` [PATCH 2/2] btrfs: Remove dead code Nikolay Borisov
2019-04-23 13:58 ` [PATCH v2] " Nikolay Borisov
2019-04-23 14:49 ` [PATCH v3] " Nikolay Borisov
2019-04-24 9:49 ` [PATCH 1/2] btrfs: Simplify snapshot exclusion code Filipe Manana
2019-04-24 10:19 ` Filipe Manana
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