linux-kernel - Re: [BUG??] Deadlock between kswapd and sys_inotify_add

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Date:	Mon, 2 Feb 2009 20:56:27 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	linux kernel <linux-kernel@...r.kernel.org>,
	linux mm <linux-mm@...ck.org>
Subject: Re: [BUG??] Deadlock between kswapd and
	sys_inotify_add_watch(lockdep  report)

Thanks for kind explanation. :)
Unfortunately, I still have a question. :(

On Mon, Feb 02, 2009 at 12:44:45PM +0100, Peter Zijlstra wrote:
> On Mon, 2009-02-02 at 20:27 +0900, MinChan Kim wrote:
> > On Mon, Feb 02, 2009 at 11:40:02AM +0100, Peter Zijlstra wrote:
> > > On Mon, 2009-02-02 at 19:25 +0900, MinChan Kim wrote:
> > > > But, I am not sure whether it's real bug or not.
> > > 
> > > Me neither, inode life-times are tricky, but on first sight it looks
> > > real enough.
> > > 
> > > > I always suffer from reading lockdep report's result. :(
> > > > It would be better to have a document about lockdep report analysis.
> > > 
> > > I've never found them hard to read, so I'm afraid you'll have to be more
> > > explicit about what is unclear to you.
> > 
> > It's becuase not lockdep humble report but my poor knowledge. :(
> > Could you elaborate please ?
> > 
> > >[  331.718120] [ INFO: inconsistent lock state ]
> > >[  331.718124] 2.6.28-rc2-mm1-lockdep #6
> > >[  331.718126] ---------------------------------
> > >[  331.718129] inconsistent {ov-reclaim-W} -> {in-reclaim-W} usage.
> >                                          ^                 ^ 
> >                                         write ?           write ?
> 
> Correct, we track states for read and write, for single state locks we
> map everything on the exclusive state (write).
> 
> > >
> > >[  331.718133] kswapd0/218 [HC0[0]:SC0[0]:HE0:SE1] takes:
> >                             ^^^^^^^^^^^^^^^^^^^^^^
> >                             what means ? HC,SC,HE,SE
> 
> Ah, yes, that's a bit obscure, but usually not needed.
> 
> Hardirq Context -- irq state tracking [preempt_count tracking]
> Softirq Context -- idem
> 
> Hardirq Enabled
> Softirq Enabled
> 
> It allows you to see if the irq state tracking matches up, and what the
> call context is.
> 
> > >
> > >[  331.718136]  (&inode->inotify_mutex){--..+.}, at: [<c01dba70>] inotify_inode_is_dead+0x20/0x90
> > >             
> > 
> > Is it related to recursive lock of inotify_mutex ?
> 
> Yes.
> 
> > but, Subject means 'inconsistent {ov-reclaim-W} -> {in-reclaim-W}', 
> > IOW, it's related to reclaim of GFP_FS. 
> > What's relation inotify_mutex and reclaim of GFP_FS?
> 
> The lockdep report states the following:
> 
> While holding inotify_mutex, we do a __GFP_FS allocation.
> But __GFP_FS allocations can end up locking inotify_mutex.
> 
> > I think if reclaim context which have GFP_FS already have lock A and then 
> > do pageout, if writepage need the lock A, we have to catch such a case. 
> > I thought Nick's patch's goal catchs such a case. 
> 
> Correct, it exactly does that.

But, I think such a case can be caught by lockdep of recursive detection 
which is existed long time ago by making you.
what's difference Nick's patch and recursive lockdep ?

-- 
Kinds Regards
MinChan Kim

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