From: Jan Kara <jack@suse.cz>
Subject: Re: next-20090310: ext4 hangs
Date: Tue, 31 Mar 2009 12:01:51 +0200
Message-ID: <20090331100150.GF11808@duck.suse.cz>
References: <a4423d670903110907o7041f4eof92278835aa0c71c@mail.gmail.com> <20090325151122.GA14881@atrey.karlin.mff.cuni.cz> <20090325151516.GB14881@atrey.karlin.mff.cuni.cz> <a4423d670903250818n512b1c67ja1b38db6d09ac7c@mail.gmail.com> <20090325152234.GN23439@duck.suse.cz> <a4423d670903250829w5e9304a7rce24f4220379620f@mail.gmail.com> <20090325161556.GP23439@duck.suse.cz> <a4423d670903251007r72058ca8m28275c1433ab2706@mail.gmail.com> <20090325194316.GQ23439@duck.suse.cz> <a4423d670903251538p3a7c180br1bcfe08c4f044c15@mail.gmail.com>
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Cc: Theodore Tso <tytso@mit.edu>,
	"linux-next@vger.kernel.org" <linux-next@vger.kernel.org>,
	linux-ext4@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	sparclinux@vger.kernel.org
To: Alexander Beregalov <a.beregalov@gmail.com>
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On Thu 26-03-09 01:38:32, Alexander Beregalov wrote:
> 2009/3/25 Jan Kara <jack@suse.cz>:
> > On Wed 25-03-09 20:07:46, Alexander Beregalov wrote:
> >> 2009/3/25 Jan Kara <jack@suse.cz>:
> >> > On Wed 25-03-09 18:29:10, Alexander Beregalov wrote:
> >> >> 2009/3/25 Jan Kara <jack@suse.cz>:
> >> >> > On Wed 25-03-09 18:18:43, Alexander Beregalov wrote:
> >> >> >> 2009/3/25 Jan Kara <jack@suse.cz>:
> >> >> >> >> > So, I think I need to try it on 2.6.29-rc7 again.
> >> >> >> >> =A0 I've looked into this. Obviously, what's happenning i=
s that we delete
> >> >> >> >> an inode and jbd2_journal_release_jbd_inode() finds inode=
 is just under
> >> >> >> >> writeout in transaction commit and thus it waits. But it =
gets never woken
> >> >> >> >> up and because it has a handle from the transaction, ever=
y one eventually
> >> >> >> >> blocks on waiting for a transaction to finish.
> >> >> >> >> =A0 But I don't really see how that can happen. The code =
is really
> >> >> >> >> straightforward and everything happens under j_list_lock.=
=2E. Strange.
> >> >> >> > =A0BTW: Is the system SMP?
> >> >> >> No, it is UP system.
> >> >> > =A0Even stranger. And do you have CONFIG_PREEMPT set?
> >> >> >
> >> >> >> The bug exists even in 2.6.29, I posted it with a new topic.
> >> >> > =A0OK, I've sort-of expected this.
> >> >>
> >> >> CONFIG_PREEMPT_RCU=3Dy
> >> >> CONFIG_PREEMPT_RCU_TRACE=3Dy
> >> >> # CONFIG_PREEMPT_NONE is not set
> >> >> # CONFIG_PREEMPT_VOLUNTARY is not set
> >> >> CONFIG_PREEMPT=3Dy
> >> >> CONFIG_DEBUG_PREEMPT=3Dy
> >> >> # CONFIG_PREEMPT_TRACER is not set
> >> >>
> >> >> config is attached.
> >> > =A0Thanks for the data. I still don't see how the wakeup can get=
 lost. The
> >> > process even cannot be preempted when we are in the section prot=
ected by
> >> > j_list_lock... Can you send me a disassembly of functions
> >> > jbd2_journal_release_jbd_inode() and journal_submit_data_buffers=
() so that
> >> > I can see whether the compiler has not reordered something unexp=
ectedly?
> > =A0Thanks for the disassembly...
> >
> >> By default gcc inlines journal_submit_data_buffers()
> >> Here is -fno-inline version. Default version is in attach.
  <snip>

  I'm helpless here. I don't see how we can miss a wakeup (plus you see=
m to
be the only one reporting the bug). Could you please compile and test t=
he kernel
with the attached patch? It will print to kernel log when we go to slee=
p
waiting for inode commit and when we send wakeups etc. When you hit the
deadlock, please send me your kernel log. It should help with debugging=
 why do
we miss the wakeup. Thanks.

									Honza
--=20
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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