linux-kernel - Re: [PATCH 1/4] mm/rmap: per anon

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Message-ID: <20131101101514.GD19466@laptop.lan>
Date:	Fri, 1 Nov 2013 11:15:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/4] mm/rmap: per anon_vma lock

On Fri, Nov 01, 2013 at 06:07:07PM +0800, Yuanhan Liu wrote:
> > I also want to point out that lately we've seen several changes sent
> > out that relax locking with no accompanying explanation of why the
> > relaxed locking would be safe. Please don't do that - having a lot of
> > performance data is worthless if you can't explain why the new locking
> > is safe.
> 
> Agreed.
> 
> > And I'm not asking to prove a negative ('lack of any possible
> > races') there, but at least in this case one could dig out why the
> > root anon vma locking was introduced and if they think that this
> > reason doesn't apply anymore, explain why...
> 
> It was introduced by commit 2b575eb6(And, BTW, I'm sorry that this commit log
> about bb4aa39676f is wrong)
> 
>    commit 2b575eb64f7a9c701fb4bfdb12388ac547f6c2b6
>    Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>    Date:   Tue May 24 17:12:11 2011 -0700
>    
>        mm: convert anon_vma->lock to a mutex
>    
>        Straightforward conversion of anon_vma->lock to a mutex.
>    
> As you can see, Peter didn't tell why before. Honestly speaking, that
> was my originaly concern as well. I tried to find some possible races;
> I guess I may miss something.

Bullshit; I didn't change the locking. I only changed the lock primitive
from a spinlock to a mutex. The anon_vma->root->lock is completely
unrelated to this change.
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