From: Jan Kara <jack@suse.cz>
To: Jeff Moyer <jmoyer@redhat.com>
Cc: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
linux-fsdevel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
Jens Axboe <jaxboe@fusionio.com>,
mgalbraith@suse.com, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Deadlocks due to per-process plugging
Date: Wed, 11 Jul 2012 22:16:01 +0200 [thread overview]
Message-ID: <20120711201601.GB9779@quack.suse.cz> (raw)
In-Reply-To: <x49ehoii8ps.fsf@segfault.boston.devel.redhat.com>
On Wed 11-07-12 12:05:51, Jeff Moyer wrote:
> Jan Kara <jack@suse.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@suse.cz>
SUSE Labs, CR
next prev parent reply other threads:[~2012-07-11 20:16 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-11 13:37 Deadlocks due to per-process plugging Jan Kara
2012-07-11 16:05 ` Jeff Moyer
2012-07-11 20:16 ` Jan Kara [this message]
2012-07-11 22:12 ` Thomas Gleixner
2012-07-12 4:12 ` Mike Galbraith
2012-07-13 12:38 ` Jan Kara
2012-07-12 2:07 ` Mike Galbraith
2012-07-12 14:15 ` Thomas Gleixner
2012-07-13 12:33 ` Jan Kara
2012-07-13 14:25 ` Thomas Gleixner
2012-07-13 14:46 ` Jan Kara
2012-07-15 8:59 ` Thomas Gleixner
2012-07-15 9:14 ` Mike Galbraith
2012-07-15 9:51 ` Thomas Gleixner
2012-07-16 2:22 ` Mike Galbraith
2012-07-16 8:59 ` Thomas Gleixner
2012-07-16 9:48 ` Mike Galbraith
2012-07-16 9:59 ` Thomas Gleixner
2012-07-16 10:13 ` Mike Galbraith
2012-07-16 10:08 ` Mike Galbraith
2012-07-16 10:19 ` Thomas Gleixner
2012-07-16 10:30 ` Mike Galbraith
2012-07-16 11:24 ` Mike Galbraith
2012-07-16 14:35 ` Mike Galbraith
2012-07-17 13:10 ` Mike Galbraith
2012-07-18 4:44 ` Mike Galbraith
2012-07-18 5:30 ` Mike Galbraith
2012-07-21 7:47 ` Mike Galbraith
2012-07-22 18:43 ` Mike Galbraith
2012-07-23 9:46 ` Mike Galbraith
2012-07-14 11:00 ` Mike Galbraith
2012-07-14 11:06 ` Mike Galbraith
2012-07-15 7:14 ` Mike Galbraith
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20120711201601.GB9779@quack.suse.cz \
--to=jack@suse.cz \
--cc=jaxboe@fusionio.com \
--cc=jmoyer@redhat.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mgalbraith@suse.com \
--cc=tglx@linutronix.de \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).