linux-kernel - Re: [NFS] 2.6.23-rc1-mm2

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Date:	Fri, 3 Aug 2007 21:21:37 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Dietrich <Marc.Dietrich@...physik.uni-giessen.de>,
	Neil Brown <neilb@...e.de>,
	Johannes Berg <johannes@...solutions.net>,
	nfs@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [NFS] 2.6.23-rc1-mm2

On 08/03, Trond Myklebust wrote:
>
> On Fri, 2007-08-03 at 09:38 -0700, Andrew Morton wrote:
> > > stack backtrace:
> > >  [<c0104fda>] show_trace_log_lvl+0x1a/0x30
> > >  [<c0105c02>] show_trace+0x12/0x20
> > >  [<c0105d15>] dump_stack+0x15/0x20
> > >  [<c013ee42>] __lock_acquire+0xc22/0x1030
> > >  [<c013f2b1>] lock_acquire+0x61/0x80
> > >  [<c012edd9>] flush_workqueue+0x49/0x70
> > >  [<c012ee0d>] flush_scheduled_work+0xd/0x10
> > >  [<dcf55c0c>] nfs_release_automount_timer+0x2c/0x30 [nfs]
> > >  [<dcf45d8e>] nfs_free_server+0x9e/0xd0 [nfs]
> > >  [<dcf4e626>] nfs_kill_super+0x16/0x20 [nfs]
> > >  [<c017b38d>] deactivate_super+0x7d/0xa0
> > >  [<c018f94b>] mntput_no_expire+0x4b/0x80
> > >  [<c018fd94>] expire_mount_list+0xe4/0x140
> > >  [<c0191219>] mark_mounts_for_expiry+0x99/0xb0
> > >  [<dcf55d1d>] nfs_expire_automounts+0xd/0x40 [nfs]
> > >  [<c012e61b>] run_workqueue+0x12b/0x1e0
> > >  [<c012f05b>] worker_thread+0x9b/0x100
> > >  [<c0131c72>] kthread+0x42/0x70
> > >  [<c0104c0f>] kernel_thread_helper+0x7/0x18
> > >  =======================
> > > 
> > 
> > There is new debugging stuff in -mm: deadlockable usage of workqueue
> > primitives will now trigger lockdep warnings.  See
> > 
> > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.23-rc1/2.6.23-rc1-mm2/broken-out/workqueue-debug-flushing-deadlocks-with-lockdep.patch
> > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.23-rc1/2.6.23-rc1-mm2/broken-out/workqueue-debug-work-related-deadlocks-with-lockdep.patch
> > 
> > I am suspecting that running flush_scheduled_work() from within run_workqueue()
> > isn't good.
> 
> I'll have a look at this. I suspect that most if not all of our calls to
> run_workqueue()/flush_scheduled_work() can now be replaced by more
> targeted calls to cancel_work_sync() and cancel_delayed_work_sync().

Yes, please, if possible.

To avoid a possible confusion: it is still OK if work->func() flushes
its own workqueue, so strictly speaking this trace is false positive,
but it would be very nice if we can get rid of this practice.

Oleg.

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