From: liubo <liubo2009@cn.fujitsu.com>
To: Sage Weil <sage@newdream.net>
Cc: Linux Btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 1/6] Btrfs: fix deadlock in btrfs_commit_transaction
Date: Tue, 26 Oct 2010 14:46:08 +0800 [thread overview]
Message-ID: <4CC67930.4030406@cn.fujitsu.com> (raw)
In-Reply-To: <1288033662-21464-2-git-send-email-sage@newdream.net>
On 10/26/2010 03:07 AM, Sage Weil wrote:
> We calculate timeout (either 1 or MAX_SCHEDULE_TIMEOUT) based on whether
> num_writers > 1 or should_grow at the top of the loop. Then, much much
> later, we wait for that timeout if either num_writers or should_grow is
> true. However, it's possible for a racing process (calling
> btrfs_end_transaction()) to decrement num_writers such that we wait
> forever instead of for 1.
>
IMO, there still exists a deadlock with your patch.
===
with your patch:
thread 1: thread 2:
btrfs_commit_transaction()
if (num_writers > 1)
timeout = MAX_TIMEOUT;
--------->
__btrfs_end_transaction()
num_writers--;
if (wq)
wake_up();
<---------
smp_mb();
prepare_wait();
if (num_writers > 1)
schedule_timeout(MAX);
else if (should_grow)
schedule_timeout(1);
===
thread2 also needs a memory_barrier, for without memory_barrier,
on some CPUs, "if (wq)" may be executed before num_writers--, like
===
thread 1: thread 2:
btrfs_commit_transaction()
if (num_writers > 1)
timeout = MAX_TIMEOUT;
--------->
__btrfs_end_transaction()
if (wq)
wake_up();
<---------
smp_mb();
prepare_wait();
if (num_writers > 1)
schedule_timeout(MAX);
else if (should_grow)
schedule_timeout(1);
---------->
num_writers--;
===
then, thread1 may wait forever.
Since wake_up() itself provides a implied wmb, and a wq active check,
it is better to drop "if (wq)" in __btrfs_end_transaction().
thanks,
liubo
> Fix this by deciding how long to wait when we wait.
>
> Signed-off-by: Sage Weil <sage@newdream.net>
> ---
> fs/btrfs/transaction.c | 12 ++++--------
> 1 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 66e4c66..bf399ea 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -992,7 +992,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> struct btrfs_root *root)
> {
> unsigned long joined = 0;
> - unsigned long timeout = 1;
> struct btrfs_transaction *cur_trans;
> struct btrfs_transaction *prev_trans = NULL;
> DEFINE_WAIT(wait);
> @@ -1063,11 +1062,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> snap_pending = 1;
>
> WARN_ON(cur_trans != trans->transaction);
> - if (cur_trans->num_writers > 1)
> - timeout = MAX_SCHEDULE_TIMEOUT;
> - else if (should_grow)
> - timeout = 1;
> -
> mutex_unlock(&root->fs_info->trans_mutex);
>
> if (flush_on_commit || snap_pending) {
> @@ -1089,8 +1083,10 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> TASK_UNINTERRUPTIBLE);
>
> smp_mb();
> - if (cur_trans->num_writers > 1 || should_grow)
> - schedule_timeout(timeout);
> + if (cur_trans->num_writers > 1)
> + schedule_timeout(MAX_SCHEDULE_TIMEOUT);
> + else if (should_grow)
> + schedule_timeout(1);
>
> mutex_lock(&root->fs_info->trans_mutex);
> finish_wait(&cur_trans->writer_wait, &wait);
next prev parent reply other threads:[~2010-10-26 6:46 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-10-25 19:07 [PATCH 0/6] Btrfs commit fixes, async subvol operations Sage Weil
2010-10-25 19:07 ` [PATCH 1/6] Btrfs: fix deadlock in btrfs_commit_transaction Sage Weil
2010-10-25 19:07 ` [PATCH 2/6] Btrfs: async transaction commit Sage Weil
2010-10-25 19:07 ` [PATCH 3/6] Btrfs: add START_SYNC, WAIT_SYNC ioctls Sage Weil
2010-10-25 19:07 ` [PATCH 4/6] Btrfs: add SNAP_CREATE_ASYNC ioctl Sage Weil
2010-10-25 19:07 ` [PATCH 5/6] Btrfs: make SNAP_DESTROY async Sage Weil
2010-10-25 19:07 ` [PATCH 6/6] Btrfs: allow subvol deletion by owner Sage Weil
2010-10-26 6:46 ` liubo [this message]
2010-10-26 16:36 ` [PATCH 1/6] Btrfs: fix deadlock in btrfs_commit_transaction Sage Weil
2010-10-26 17:06 ` Chris Mason
2010-10-27 0:41 ` liubo
2010-10-25 19:29 ` [PATCH 0/6] Btrfs commit fixes, async subvol operations Chris Mason
2010-10-25 19:41 ` Sage Weil
2010-10-25 19:58 ` Chris Mason
2010-10-25 21:27 ` Sage Weil
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