linux-kernel - Re: Deadlocks due to per-process plugging

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Date:	Wed, 11 Jul 2012 22:16:01 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Jens Axboe <jaxboe@...ionio.com>, mgalbraith@...e.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Deadlocks due to per-process plugging

On Wed 11-07-12 12:05:51, Jeff Moyer wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> >   Hello,
> >
> >   we've recently hit a deadlock in our QA runs which is caused by the
> > per-process plugging code. The problem is as follows:
> >   process A					process B (kjournald)
> >   generic_file_aio_write()
> >     blk_start_plug(&plug);
> >     ...
> >     somewhere in here we allocate memory and
> >     direct reclaim submits buffer X for IO
> >     ...
> >     ext3_write_begin()
> >       ext3_journal_start()
> >         we need more space in a journal
> >         so we want to checkpoint old transactions,
> >         we block waiting for kjournald to commit
> >         a currently running transaction.
> > 						journal_commit_transaction()
> > 						  wait for IO on buffer X
> > 						  to complete as it is part
> > 						  of the current transaction
> >
> >   => deadlock since A waits for B and B waits for A to do unplug.
> > BTW: I don't think this is really ext3/ext4 specific. I think other
> > filesystems can get into problems as well when direct reclaim submits some
> > IO and the process subsequently blocks without submitting the IO.
> 
> So, I thought schedule would do the flush.  Checking the code:
> 
> asmlinkage void __sched schedule(void)
> {
>         struct task_struct *tsk = current;
> 
>         sched_submit_work(tsk);
>         __schedule();
> }
> 
> And sched_submit_work looks like this:
> 
> static inline void sched_submit_work(struct task_struct *tsk)
> {
>         if (!tsk->state || tsk_is_pi_blocked(tsk))
>                 return;
>         /*
>          * If we are going to sleep and we have plugged IO queued,
>          * make sure to submit it to avoid deadlocks.
>          */
>         if (blk_needs_flush_plug(tsk))
>                 blk_schedule_flush_plug(tsk);
> }
> 
> This eventually ends in a call to blk_run_queue_async(q) after
> submitting the I/O from the plug list.  Right?  So is the question
> really why doesn't the kblockd workqueue get scheduled?
  Ah, I didn't know this. Thanks for the hint. So in the kdump I have I can
see requests queued in tsk->plug despite the process is sleeping in
TASK_UNINTERRUPTIBLE state.  So the only way how unplug could have been
omitted is if tsk_is_pi_blocked() was true. Rummaging through the dump...
indeed task has pi_blocked_on = 0xffff8802717d79c8. The dump is from an -rt
kernel (I just didn't originally thought that makes any difference) so
actually any mutex is rtmutex and thus tsk_is_pi_blocked() is true whenever
we are sleeping on a mutex. So this seems like a bug in rtmutex code.
Thomas, you seemed to have added that condition... Any idea how to avoid
the deadlock?

									Honza 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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