linux-kernel - Re: [RFC PATCH] shrink_dcache

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Message-ID: <CAJfpegszrDO112d9A3PLsMY1+NnRpqFoLzODABnMq_VZq=DWFw@mail.gmail.com>
Date:	Tue, 10 Jan 2012 17:22:22 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	mgorman@...e.de, gregkh@...e.de, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH] shrink_dcache_parent() deadlock

On Tue, Jan 10, 2012 at 5:00 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Jan 10, 2012 at 2:05 AM, Miklos Szeredi <miklos@...redi.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.

Thanks,
Miklos
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