linux-ext4 - Re: [PATCH] jbd2: Fix forever sleeping process in do_get_write

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Date:	Thu, 05 May 2011 08:49:14 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	tytso@....edu, Tao Ma <tm@....ma>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] jbd2: Fix forever sleeping process in do_get_write_access()

On 5/5/11 7:10 AM, Jan Kara wrote:
> In do_get_write_access() we wait on BH_Unshadow bit for buffer to get
> from shadow state. The waking code in journal_commit_transaction() has
> a bug because it does not issue a memory barrier after the buffer is moved
> from the shadow state and before wake_up_bit() is called. Thus a waitqueue
> check can happen before the buffer is actually moved from the shadow state
> and waiting process may never be woken. Fix the problem by issuing proper
> barrier.

needed for jbd/commit.c as well, I guess?

-Eric

> Reported-by: Tao Ma <boyu.mt@...bao.com>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/jbd2/commit.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
>  Analogous JBD fix has been queued in my tree...
> 
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 2e5d370..3a958c7 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -768,8 +768,13 @@ wait_for_iobuf:
>                     required. */
>  		JBUFFER_TRACE(jh, "file as BJ_Forget");
>  		jbd2_journal_file_buffer(jh, commit_transaction, BJ_Forget);
> -		/* Wake up any transactions which were waiting for this
> -		   IO to complete */
> +		/*
> +		 * Wake up any transactions which were waiting for this IO to
> +		 * complete. The barrier must be here so that changes by
> +		 * jbd2_journal_file_buffer() take effect before wake_up_bit()
> +		 * does the waitqueue check.
> +		 */
> +		smp_mb();
>  		wake_up_bit(&bh->b_state, BH_Unshadow);
>  		JBUFFER_TRACE(jh, "brelse shadowed buffer");
>  		__brelse(bh);

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