Re: [PATCH 1/4] mm/rmap: per anon_vma lock

[Peter Zijlstra <peterz@infradead.org>]

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@chello.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.