linux-kernel - Re: [BUGFIX v2] memcg: fix res_counter_read_u64 lock aware (Was Re: [PATCH] oom: handle overflow in mem_cgroup_out_of

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Message-Id: <20110127111350.cc5b3111.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 27 Jan 2011 11:13:50 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>
Subject: Re: [BUGFIX v2] memcg: fix res_counter_read_u64 lock aware (Was Re:
 [PATCH] oom: handle overflow in mem_cgroup_out_of_memory()

On Wed, 26 Jan 2011 17:57:22 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 27 Jan 2011 10:43:39 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > --- mmotm-0125.orig/kernel/res_counter.c
> > +++ mmotm-0125/kernel/res_counter.c
> > @@ -126,10 +126,24 @@ ssize_t res_counter_read(struct res_coun
> >  			pos, buf, s - buf);
> >  }
> >  
> > +#if BITS_PER_LONG == 32
> > +u64 res_counter_read_u64(struct res_counter *counter, int member)
> > +{
> > +	unsigned long flags;
> > +	u64 ret;
> > +
> > +	spin_lock_irqsave(&counter->lock, flags);
> > +	ret = *res_counter_member(counter, member);
> > +	spin_unlock_irqrestore(&counter->lock, flags);
> > +
> > +	return ret;
> > +}
> > +#else
> >  u64 res_counter_read_u64(struct res_counter *counter, int member)
> >  {
> >  	return *res_counter_member(counter, member);
> >  }
> > +#endif
> 
> _irqsave is only needed if the lock will be taken from irq context. 
> Does that happen?
> 
I just obey current desing of res_counter, as bugfix.
This counter is designed to be safe against irq context.
Adding CC: to Balbir.

To be honest, it has never happened since res_counter is introduced. I imagine
there was a big plan when this counter was designed. But I think it will 
be never called other than memcg because cpu, blkio controller haven't
use res_counter, finally. And memcg tends to use per-cpu counter because of
performance.

If I need to remove irq flags from this function, I'll do in another patch
which changes total design of res_counter and make it not safe agaisnt irq context.

Thanks,
-Kame

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