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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Jun 2011 14:26:41 -0700
Message-ID: <BANLkTimLV8aCZ7snXT_Do+f4vRY0EkoS4A@mail.gmail.com>
Subject: Re: REGRESSION: Performance regressions from switching anon_vma->lock
 to mutex
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Andi Kleen <ak@linux.intel.com>,
        Shaohua Li <shaohua.li@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        David Miller <davem@davemloft.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Russell King <rmk@arm.linux.org.uk>, Paul Mundt <lethal@linux-sh.org>,
        Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>,
        "Luck, Tony" <tony.luck@intel.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>,
        Namhyung Kim <namhyung@gmail.com>, "Shi, Alex" <alex.shi@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Rafael J. Wysocki" <rjw@sisk.pl>
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On Thu, Jun 16, 2011 at 2:06 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jun 16, 2011 at 2:05 PM, Linus Torvalds
> <torvalds@linux-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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