linux-kernel - Re: /proc dcache deadlock in do

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Message-ID: <m1zlwzgn6h.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 27 Nov 2007 18:43:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <andrea@...e.de>, linux-kernel@...r.kernel.org,
	jack@...e.cz, Ingo Molnar <mingo@...e.hu>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: /proc dcache deadlock in do_exit

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Tue, 27 Nov 2007 14:20:22 +0100
> Andrea Arcangeli <andrea@...e.de> wrote:
>
>> Hi,
>> 
>> this patch fixes a sles9 system hang in start_this_handle from a
>> customer with some heavy workload where all tasks are waiting on
>> kjournald to commit the transaction, but kjournald waits on t_updates
>> to go down to zero (it never does). This was reported as a lowmem
>> shortage deadlock but when checking the debug data I noticed the VM
>> wasn't under pressure at all (well it was really under vm pressure,
>> because lots of tasks hanged in the VM prune_dcache methods trying to
>> flush dirty inodes, but no task was hanging in GFP_NOFS mode, the
>> holder of the journal handle should have if this was a vm issue in the
>> first place). No task was apparently holding the leftover handle in
>> the committing transaction, so I deduced t_updates was stuck to 1
>> because a journal_stop was never run by some path (this turned out to
>> be correct). With a debug patch adding proper reverse links and stack
>> trace logging in ext3 deployed in production, I found journal_stop is
>> never run because mark_inode_dirty_sync is called inside release_task
>> called by do_exit. (that was quite fun because I would have never
>> thought about this subtleness, I thought a regular path in ext3 had a
>> bug and it forgot to call journal_stop)
>> 
>> do_exit->release_task->mark_inode_dirty_sync->schedule() (will never
>> come back to run journal_stop)
>
> I don't see why the schedule() will not return?  Because the task has
> PF_EXITING set?  Doesn't TASK_DEAD do that?

Yes, why do we not come back from schedule?

If we are not allowed to schedule after setting PF_EXITING before
we set TASK_DEAD that entire code path sounds brittle and
error prone.


> What are the implications of not running shrink_dcache_parent() on the exit
> path sometimes?  We'll leave procfs stuff behind?  Will they be reaped by
> memory pressure later on?

It should.  I think the reaping is just an optimization.  Because we
know we will never need those dentries again, and we can pin them by
open directories or opening files.  What I don't know off the top of
my head is if there is a d_drop equivalent going on that might be a
problem if we don't address it.

Eric

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