linux-kernel - Re: REGRESSION: Performance regressions from switching anon

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Message-ID: <BANLkTimLV8aCZ7snXT_Do+f4vRY0EkoS4A@mail.gmail.com>
Date:	Thu, 16 Jun 2011 14:26:41 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andi Kleen <ak@...ux.intel.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	"Luck, Tony" <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Namhyung Kim <namhyung@...il.com>,
	"Shi, Alex" <alex.shi@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: REGRESSION: Performance regressions from switching anon_vma->lock
 to mutex

On Thu, Jun 16, 2011 at 2:06 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jun 16, 2011 at 2:05 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> This patch is UNTESTED!
>
> It was also UNATTACHED!

Hmm. And it doesn't work. We deadlock when we free the anon_vma
because the *freeing* path wants to take the anon_vma lock. See that
horrid code in anon_vma_free().

So now we now hold the root over the whole series of frees, and get an
instant deadlock.

We also can happen to free the root anon_vma before we release the
lock in it, which is another slight problem ;)

So the unlink_anon_vmas() case is actually much more complicated than
the clone case.

In other words, just forget that second patch. I'll have to think about it.

                              Linus
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