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Message-ID: <20070816023558.GE14613@linux.vnet.ibm.com>
Date: Wed, 15 Aug 2007 19:35:58 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Paul Mackerras <paulus@...ba.org>,
Christoph Lameter <clameter@....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 10:11:05AM +0800, Herbert Xu wrote:
> 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.
If the load-coalescing is important to performance, why not load into
a local variable?
Thanx, Paul
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