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Date: Wed, 11 Jan 2012 08:52:40 +1100
From: Dave Chinner <david@fromorbit.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, mgorman@suse.de, gregkh@suse.de,
        akpm@linux-foundation.org
Subject: Re: [RFC PATCH] shrink_dcache_parent() deadlock
Message-ID: <20120110215240.GA24410@dastard>
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On Tue, Jan 10, 2012 at 05:22:22PM +0100, Miklos Szeredi wrote:
> On Tue, Jan 10, 2012 at 5:00 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > On Tue, Jan 10, 2012 at 2:05 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >>
> >> I tested Dave's patch and the bug can still be easily reproduced.
> >>
> >> And that's to be expected, as the intermediate "being on the lru"
> >> state that Dave's patch eliminates doesn't play a fundamental part in
> >> the mechanism of the livelock.  It does eliminate one trylock, but
> >> that's not the one critical to this bug (removing it is a very good
> >> idea anyway).
> >>
> >> The critical trylock here is the one in dentry_kill() which tries to
> >> lock the parent.
> >
> > Ok. Here's your patch munged for current -git. You've got most of a
> > changelog, can you send this out with the right subject and a
> > sign-off, and re-test with the current git just to make sure.
> 
> See the one with the subject "vfs: fix shrink_dcache_parent()
> livelock" I sent out a bit earlier.
> 
> You didn't quite get it right: the flag now needs to be set in
> select_parent() not prune_dcache_sb().
> 
> I think prune_dcache_sb() doesn't need this logic (although it
> wouldn't hurt either) because that one is called from the slab
> shrinkers and those are protected from being run multiple times in
> parallel, I hope.

Shrinkers can be called in parallel by memory reclaim on different
CPUs. The only thing serialising them is the LRU locks.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com