From: Rik van Riel <riel@surriel.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org, kernel-team@fb.com,
linux-kernel@vger.kernel.org,
"Darrick J. Wong" <darrick.wong@oracle.com>,
David Chinner <dchinner@redhat.com>
Subject: Re: [PATCH] fs,xfs: fix missed wakeup on l_flush_wait
Date: Wed, 08 May 2019 10:08:59 -0400 [thread overview]
Message-ID: <3985b9feffe11dcdbb86fa8c2d9ffc4bd7ab8458.camel@surriel.com> (raw)
In-Reply-To: <20190507212213.GO29573@dread.disaster.area>
[-- Attachment #1: Type: text/plain, Size: 2829 bytes --]
On Wed, 2019-05-08 at 07:22 +1000, Dave Chinner wrote:
> On Tue, May 07, 2019 at 01:05:28PM -0400, Rik van Riel wrote:
> > The code in xlog_wait uses the spinlock to make adding the task to
> > the wait queue, and setting the task state to UNINTERRUPTIBLE
> > atomic
> > with respect to the waker.
> >
> > Doing the wakeup after releasing the spinlock opens up the
> > following
> > race condition:
> >
> > - add task to wait queue
> >
> > - wake up task
> >
> > - set task state to UNINTERRUPTIBLE
> >
> > Simply moving the spin_unlock to after the wake_up_all results
> > in the waker not being able to see a task on the waitqueue before
> > it has set its state to UNINTERRUPTIBLE.
>
> Yup, seems like an issue. Good find, Rik.
>
> So, what problem is this actually fixing? Was it noticed by
> inspection, or is it actually manifesting on production machines?
> If it is manifesting IRL, what are the symptoms (e.g. hang running
> out of log space?) and do you have a test case or any way to
> exercise it easily?
Chris spotted a hung kworker task, in UNINTERRUPTIBLE
state, but with an empty wait queue. This does not seem
like something that is easily reproducible.
> And, FWIW, did you check all the other xlog_wait() users for the
> same problem?
I did not, but am looking now. The xlog_wait code itself
is fine, but it seems there are a few other wakers that
are doing the wakeup after releasing the lock.
It looks like xfs_log_force_umount() and the other wakeup
in xlog_state_do_callback() suffer from the same issue.
> > The lock ordering of taking the waitqueue lock inside the
> > l_icloglock
> > is already used inside xlog_wait; it is unclear why the waker was
> > doing
> > things differently.
>
> Historical, most likely, and the wakeup code has changed in years
> gone by and a race condition that rarely manifests is unlikely to be
> noticed.
>
> ....
>
> Yeah, the conversion from counting semaphore outside the iclog lock
> to use wait queues in 2008 introduced this bug. The wait queue
> addition was moved inside the lock, the wakeup left outside. So:
It looks like that conversion may have introduced the
same bug in multiple places.
That first wakeup in xlog_state_do_callback() looks pretty
straightforward. That spin_unlock could be moved down as well,
or a lock & unlock pair could be placed around the wake_up_all.
I am not sure about xfs_log_force_umount(). Could the unlock
be moved to after the wake_up_all, or does that create lock
ordering issues with the xlog_grant_head_wake_all calls?
Could a simple lock + unlock of log->l_icloglock around the
wake_up_all do the trick, or is there some other state that
also needs to stay locked?
--
All Rights Reversed.
[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 488 bytes --]
next prev parent reply other threads:[~2019-05-08 14:09 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-07 17:05 [PATCH] fs,xfs: fix missed wakeup on l_flush_wait Rik van Riel
2019-05-07 21:22 ` Dave Chinner
2019-05-08 14:08 ` Rik van Riel [this message]
2019-05-08 21:32 ` Dave Chinner
2019-05-09 14:27 ` Rik van Riel
2019-05-08 16:39 ` Chris Mason
2019-05-08 21:40 ` Dave Chinner
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=3985b9feffe11dcdbb86fa8c2d9ffc4bd7ab8458.camel@surriel.com \
--to=riel@surriel.com \
--cc=darrick.wong@oracle.com \
--cc=david@fromorbit.com \
--cc=dchinner@redhat.com \
--cc=kernel-team@fb.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-xfs@vger.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).