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Date:	Fri, 5 Jun 2009 13:06:56 -0600
From:	"Trenton D. Adams" <trenton.d.adams@...il.com>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Reiserfs <reiserfs-devel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alexander Beregalov <a.beregalov@...il.com>
Subject: Re: [PATCH 1/2] kill-the-bkl/reiserfs: acquire the inode mutex safely

On Fri, Jun 5, 2009 at 12:26 PM, Jeff Mahoney<jeffm@...e.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Trenton D. Adams wrote:
>> On Sat, May 16, 2009 at 12:02 PM, Frederic Weisbecker
>> <fweisbec@...il.com> wrote:
>>> While searching a pathname, an inode mutex can be acquired
>>> in do_lookup() which calls reiserfs_lookup() which in turn
>>> acquires the write lock.
>>>
>>> On the other side reiserfs_fill_super() can acquire the write_lock
>>> and then call reiserfs_lookup_privroot() which can acquire an
>>> inode mutex (the root of the mount point).
>>>
>>> So we theoretically risk an AB - BA lock inversion that could lead
>>> to a deadlock.
>>>
>>> As for other lock dependencies found since the bkl to mutex
>>> conversion, the fix is to use reiserfs_mutex_lock_safe() which
>>> drops the lock dependency to the write lock.
>>>
>>
>> I'm curious, did this get applied, and is it related to the following?
>>  I was having these in 2.6.30-rc3.  I am now on 2.6.30-rc7 as of
>> today.  I haven't seen them today.  But then again, I only seen this
>> happen one time.
>>
>> May 27 01:56:12 tdamac INFO: task pdflush:15370 blocked for more than
>> 120 seconds.
>> May 27 01:56:12 tdamac "echo 0 >
>> /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> May 27 01:56:12 tdamac pdflush       D ffff8800518a0000     0 15370      2
>> May 27 01:56:12 tdamac ffff880025023b50 0000000000000046
>> 0000000025023a90 000000000000d7a0
>> May 27 01:56:12 tdamac 0000000000004000 0000000000011440
>> 000000000000ca78 ffff880045e71568
>> May 27 01:56:12 tdamac ffff880045e7156c ffff8800518a0000
>> ffff880067f54230 ffff8800518a0380
>> May 27 01:56:12 tdamac Call Trace:
>> May 27 01:56:12 tdamac [<ffffffff80687d1b>] ? __mutex_lock_slowpath+0xe2/0x124
>> May 27 01:56:12 tdamac [<ffffffff80687d13>] __mutex_lock_slowpath+0xda/0x124
>> May 27 01:56:12 tdamac [<ffffffff8068809e>] mutex_lock+0x1e/0x36
>> May 27 01:56:12 tdamac [<ffffffff803087ae>] flush_commit_list+0x150/0x689
>> May 27 01:56:12 tdamac [<ffffffff8022f8e5>] ? __wake_up+0x43/0x50
>> May 27 01:56:12 tdamac [<ffffffff8030ad8a>] do_journal_end+0xb4a/0xd6c
>> May 27 01:56:12 tdamac [<ffffffff8023053d>] ? dequeue_entity+0x1b/0x1df
>> May 27 01:56:12 tdamac [<ffffffff8030b020>] journal_end_sync+0x74/0x7d
>> May 27 01:56:12 tdamac [<ffffffff802fd2fd>] reiserfs_sync_fs+0x41/0x67
>> May 27 01:56:12 tdamac [<ffffffff80688091>] ? mutex_lock+0x11/0x36
>> May 27 01:56:12 tdamac [<ffffffff802fd331>] reiserfs_write_super+0xe/0x10
>> May 27 01:56:12 tdamac [<ffffffff802a532a>] sync_supers+0x61/0xa6
>> May 27 01:56:12 tdamac [<ffffffff8027e140>] wb_kupdate+0x32/0x128
>> May 27 01:56:12 tdamac [<ffffffff8027ee7c>] pdflush+0x140/0x21f
>> May 27 01:56:12 tdamac [<ffffffff8027e10e>] ? wb_kupdate+0x0/0x128
>> May 27 01:56:12 tdamac [<ffffffff8027ed3c>] ? pdflush+0x0/0x21f
>> May 27 01:56:12 tdamac [<ffffffff8024fb26>] kthread+0x56/0x83
>> May 27 01:56:12 tdamac [<ffffffff8020beba>] child_rip+0xa/0x20
>> May 27 01:56:12 tdamac [<ffffffff8024fad0>] ? kthread+0x0/0x83
>> May 27 01:56:12 tdamac [<ffffffff8020beb0>] ? child_rip+0x0/0x20
>
> Can you capture a sysrq+t when this happens? The lock is properly
> released, but I have a hunch that another thread is doing ordered
> writeback that's taking a while. That happens under the j_commit_mutex.

FYI: I never did anything specific that I knew of, so I didn't
actually notice a delay.  I was rsyncing to a USB key at the time.
And seeing it took over an hour, I walked away, so I wouldn't have
noticed it.  But, I could fiddle around a little to see if I could get
some sort of delay going on.  Any ideas on what I should try?  Then I
can do the sysreq+t for you if I can reproduce.
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