linux-kernel - Re: [patch] mm, mempolicy: Introduce spinlock to read shared policy tree

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Message-ID: <CA+55aFxrdPpMWLD8LF0NNqgJqmB-L-HW3Xyxht6e5AwnoaueTw@mail.gmail.com>
Date:	Fri, 21 Dec 2012 08:53:33 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	David Rientjes <rientjes@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sasha Levin <levinsasha928@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [patch] mm, mempolicy: Introduce spinlock to read shared policy tree

On Fri, Dec 21, 2012 at 5:47 AM, Mel Gorman <mgorman@...e.de> wrote:
> On Thu, Dec 20, 2012 at 02:55:22PM -0800, David Rientjes wrote:
>>
>> This is probably worth discussing now to see if we can't revert
>> b22d127a39dd ("mempolicy: fix a race in shared_policy_replace()"), keep it
>> only as a spinlock as you suggest, and do what KOSAKI suggested in
>> http://marc.info/?l=linux-kernel&m=133940650731255 instead.  I don't think
>> it's worth trying to optimize this path at the cost of having both a
>> spinlock and mutex.
>
> Jeez, I'm still not keen on that approach for the reasons that are explained
> in the changelog for b22d127a39dd.

Christ, Mel.

Your reasons in b22d127a39dd are weak as hell, and then you come up
with *THIS* shit instead:

> That leads to this third *ugly* option that conditionally drops the lock
> and it's up to the caller to figure out what happened. Fooling around with
> how it conditionally releases the lock results in different sorts of ugly.
> We now have three ugly sister patches for this. Who wants to be Cinderalla?
>
> ---8<---
> mm: numa: Release the PTL if calling vm_ops->get_policy during NUMA hinting faults

Heck no. In fact, not a f*cking way in hell. Look yourself in the
mirror, Mel. This patch is ugly, and *guaranteed* to result in subtle
locking issues, and then you have the *gall* to quote the "uhh, that's
a bit ugly due to some trivial duplication" thing in commit
b22d127a39dd.

Reverting commit b22d127a39dd and just having a "ok, if we need to
allocate, then drop the lock, allocate, re-get the lock, and see if we
still need the new allocation" is *beautiful* code compared to the
diseased abortion you just posted.

Seriously. Conditional locking is error-prone, and about a million
times worse than the trivial fix that Kosaki suggested.

                         Linus
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