linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Date:	Thu, 16 Aug 2007 10:11:05 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Christoph Lameter <clameter@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Satyam Sharma <satyam@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Chris Snook <csnook@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Thu, Aug 16, 2007 at 12:05:56PM +1000, Paul Mackerras wrote:
> Herbert Xu writes:
> 
> > See sk_stream_mem_schedule in net/core/stream.c:
> > 
> >         /* Under limit. */
> >         if (atomic_read(sk->sk_prot->memory_allocated) < sk->sk_prot->sysctl_mem[0]) {
> >                 if (*sk->sk_prot->memory_pressure)
> >                         *sk->sk_prot->memory_pressure = 0;
> >                 return 1;
> >         }
> > 
> >         /* Over hard limit. */
> >         if (atomic_read(sk->sk_prot->memory_allocated) > sk->sk_prot->sysctl_mem[2]) {
> >                 sk->sk_prot->enter_memory_pressure();
> >                 goto suppress_allocation;
> >         }
> > 
> > We don't need to reload sk->sk_prot->memory_allocated here.
> 
> Are you sure?  How do you know some other CPU hasn't changed the value
> in between?

Yes I'm sure, because we don't care if others have increased
the reservation.

Note that even if we did we'd be using barriers so volatile
won't do us any good here.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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