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Date:	Wed, 12 Jun 2013 22:55:26 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	linux-ext4@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 2/6] jbd2/log_wait_for_space: drop checkpoint mutex when
 waiting

On Tue, Jun 11, 2013 at 06:44:26PM -0400, Paul Gortmaker wrote:
> While trying to debug an an issue under extreme I/O loading
> on preempt-rt kernels, the following backtrace was observed
> via SysRQ output:
> 
> rm              D ffff8802203afbc0  4600  4878   4748 0x00000000
>  ffff8802217bfb78 0000000000000082 ffff88021fc2bb80 ffff88021fc2bb80
>  ffff88021fc2bb80 ffff8802217bffd8 ffff8802217bffd8 ffff8802217bffd8
>  ffff88021f1d4c80 ffff88021fc2bb80 ffff8802217bfb88 ffff88022437b000
> Call Trace:
>  [<ffffffff8172dc34>] schedule+0x24/0x70
>  [<ffffffff81225b5d>] jbd2_log_wait_commit+0xbd/0x140
>  [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50
>  [<ffffffff81223635>] jbd2_log_do_checkpoint+0xf5/0x520
>  [<ffffffff81223b09>] __jbd2_log_wait_for_space+0xa9/0x1f0
>  [<ffffffff8121dc40>] start_this_handle.isra.10+0x2e0/0x530
>  [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50
>  [<ffffffff8121e0a3>] jbd2__journal_start+0xc3/0x110
>  [<ffffffff811de7ce>] ? ext4_rmdir+0x6e/0x230
>  [<ffffffff8121e0fe>] jbd2_journal_start+0xe/0x10
>  [<ffffffff811f308b>] ext4_journal_start_sb+0x5b/0x160
>  [<ffffffff811de7ce>] ext4_rmdir+0x6e/0x230
>  [<ffffffff811435c5>] vfs_rmdir+0xd5/0x140
>  [<ffffffff8114370f>] do_rmdir+0xdf/0x120
>  [<ffffffff8105c6b4>] ? task_work_run+0x44/0x80
>  [<ffffffff81002889>] ? do_notify_resume+0x89/0x100
>  [<ffffffff817361ae>] ? int_signal+0x12/0x17
>  [<ffffffff81145d85>] sys_unlinkat+0x25/0x40
>  [<ffffffff81735f22>] system_call_fastpath+0x16/0x1b
> 
> What is interesting here, is that we call log_wait_commit, from
> within wait_for_space, but we are still holding the checkpoint_mutex
> as it surrounds mostly the whole of wait_for_space.  And then, as we
> are waiting, journal_commit_transaction can run, and if the JBD2_FLUSHED
> bit is set, then we will also try to take the same checkpoint_mutex.
> 
> It seems that we need to drop the checkpoint_mutex while sitting in
> jbd2_log_wait_commit, if we want to guarantee that progress can be made
> by jbd2_journal_commit_transaction().  There does not seem to be
> anything preempt-rt specific about this, other then perhaps increasing
> the odds of it happening.
> 
> Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>

Applied, thanks.

						- Ted
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