linux-ext4 - Re: [syzbot] possible deadlock in jbd2_journal_lock

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Message-ID: <20221026101854.k6qgunxexhxthw64@quack3>
Date:   Wed, 26 Oct 2022 12:18:54 +0200
From:   Jan Kara <jack@...e.cz>
To:     Thilo Fromm <t-lo@...ux.microsoft.com>
Cc:     Jan Kara <jack@...e.cz>, Ye Bin <yebin10@...wei.com>,
        jack@...e.com, tytso@....edu, linux-ext4@...r.kernel.org,
        regressions@...ts.linux.dev,
        Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Subject: Re: [syzbot] possible deadlock in jbd2_journal_lock_updates

On Mon 24-10-22 18:32:51, Thilo Fromm wrote:
> Hello Honza,
> 
> > Yeah, I was pondering about this for some time but still I have no clue who
> > could be holding the buffer lock (which blocks the task holding the
> > transaction open) or how this could related to the commit you have
> > identified. I have two things to try:
> > 
> > 1) Can you please check whether the deadlock reproduces also with 6.0
> > kernel? The thing is that xattr handling code in ext4 has there some
> > additional changes, commit 307af6c8793 ("mbcache: automatically delete
> > entries from cache on freeing") in particular.
> 
> This would be complex; we currently do not integrate 6.0 with Flatcar and
> would need to spend quite some effort ingesting it first (mostly, make sure
> the new kernel does not break something unrelated). Flatcar is an
> image-based distro, so kernel updates imply full distro updates.

OK, understood.

> > 2) I have created a debug patch (against 5.15.x stable kernel). Can you
> > please reproduce the failure with it and post the output of "echo w
> > > /proc/sysrq-trigger" and also the output the debug patch will put into the
> > kernel log? It will dump the information about buffer lock owner if we > cannot get the lock for more than 32 seconds.
> 
> This would be more straightforward - I can reach out to one of our users
> suffering from the issue; they can reliably reproduce it and don't shy away
> from patching their kernel. Where can I find the patch?

Ha, my bad. I forgot to attach it. Here it is.

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

View attachment "0001-Debug-buffer_head-lock.patch" of type "text/x-patch" (2185 bytes)