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Message-ID: <1262887447.28171.3736.camel@gandalf.stny.rr.com>
Date: Thu, 07 Jan 2010 13:04:07 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: paulmck@...ux.vnet.ibm.com
Cc: Peter Zijlstra <peterz@...radead.org>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
akpm@...ux-foundation.org, tglx@...utronix.de,
Valdis.Kletnieks@...edu, dhowells@...hat.com, laijs@...fujitsu.com,
dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
barrier
On Thu, 2010-01-07 at 09:56 -0800, Paul E. McKenney wrote:
> On Thu, Jan 07, 2010 at 12:44:37PM -0500, Steven Rostedt wrote:
> > On Thu, 2010-01-07 at 09:31 -0800, Paul E. McKenney wrote:
> >
> > > Something like the following for sys_membarrier(), then?
> > >
> > > smp_mb();
> > > for_each_cpu(cpu, current->mm->cpu_vm_mask) {
> > > if (cpu_curr(cpu)->mm == current->mm)
> > > smp_call_function_single(cpu, func, NULL, 1);
> > > }
> > >
> > > Then the code changing ->mm on the other CPU also needs to have a
> > > full smp_mb() somewhere after the change to ->mm, but before starting
> > > user-space execution. Which it might well just due to overhead, but
> > > we need to make sure that someone doesn't optimize us out of existence.
> >
> > To change the mm requires things like flushing the TLB. I'd be surprised
> > if the change of the mm does not already do a smp_mb() somewhere.
>
> Agreed, but "somewhere" does not fill me with warm fuzzies. ;-)
Another question would be, does flushing the TLB imply a mb()?
-- Steve
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