From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Victor Kaplansky <VICTORK@il.ibm.com>,
Anton Blanchard <anton@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux PPC dev <linuxppc-dev@ozlabs.org>,
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
Michael Ellerman <michael@ellerman.id.au>,
Michael Neuling <mikey@neuling.org>,
Oleg Nesterov <oleg@redhat.com>
Subject: Re: perf events ring buffer memory barrier on powerpc
Date: Mon, 4 Nov 2013 10:07:44 +0100 [thread overview]
Message-ID: <20131104090744.GE10651@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20131102152048.GI4067@linux.vnet.ibm.com>
On Sat, Nov 02, 2013 at 08:20:48AM -0700, Paul E. McKenney wrote:
> On Fri, Nov 01, 2013 at 11:30:17AM +0100, Peter Zijlstra wrote:
> > Furthermore there's a gazillion parallel userspace programs.
>
> Most of which have very unaggressive concurrency designs.
pthread_mutex_t A, B;
char data_A[x];
int counter_B = 1;
void funA(void)
{
pthread_mutex_lock(&A);
memset(data_A, 0, sizeof(data_A));
pthread_mutex_unlock(&A);
}
void funB(void)
{
pthread_mutex_lock(&B);
counter_B++;
pthread_mutex_unlock(&B);
}
void funC(void)
{
pthread_mutex_lock(&B)
printf("%d\n", counter_B);
pthread_mutex_unlock(&B);
}
Then run: funA, funB, funC concurrently, and end with a funC.
Then explain to userman than his unaggressive program can return:
0
1
Because the memset() thought it might be a cute idea to overwrite
counter_B and fix it up 'later'. Which if I understood you right is
valid in C/C++ :-(
Not that any actual memset implementation exhibiting this trait wouldn't
be shot on the spot.
> > > By marking "ptr" as atomic, thus telling the compiler not to mess with it.
> > > And thus requiring that all accesses to it be decorated, which in the
> > > case of RCU could be buried in the RCU accessors.
> >
> > This seems contradictory; marking it atomic would look like:
> >
> > struct foo {
> > unsigned long value;
> > __atomic void *ptr;
> > unsigned long value1;
> > };
> >
> > Clearly we cannot hide this definition in accessors, because then
> > accesses to value* won't see the annotation.
>
> #define __rcu __atomic
Yeah, except we don't use __rcu all that consistently; in fact I don't
know if I ever added it.
next prev parent reply other threads:[~2013-11-04 9:08 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-23 7:39 ` Victor Kaplansky
2013-10-23 14:19 ` Frederic Weisbecker
2013-10-23 14:25 ` Frederic Weisbecker
2013-10-25 17:37 ` Peter Zijlstra
2013-10-25 20:31 ` Michael Neuling
2013-10-27 9:00 ` Victor Kaplansky
2013-10-28 9:22 ` Peter Zijlstra
2013-10-28 10:02 ` Frederic Weisbecker
2013-10-28 12:38 ` Victor Kaplansky
2013-10-28 13:26 ` Peter Zijlstra
2013-10-28 16:34 ` Paul E. McKenney
2013-10-28 20:17 ` Oleg Nesterov
2013-10-28 20:58 ` Victor Kaplansky
2013-10-29 10:21 ` Peter Zijlstra
2013-10-29 10:30 ` Peter Zijlstra
2013-10-29 10:35 ` Peter Zijlstra
2013-10-29 20:15 ` Oleg Nesterov
2013-10-29 19:27 ` Vince Weaver
2013-10-30 10:42 ` Peter Zijlstra
2013-10-30 11:48 ` James Hogan
2013-10-30 12:48 ` Peter Zijlstra
2013-11-06 13:19 ` [tip:perf/core] tools/perf: Add required memory barriers tip-bot for Peter Zijlstra
2013-11-06 13:50 ` Vince Weaver
2013-11-06 14:00 ` Peter Zijlstra
2013-11-06 14:28 ` Peter Zijlstra
2013-11-06 14:55 ` Vince Weaver
2013-11-06 15:10 ` Peter Zijlstra
2013-11-06 15:23 ` Peter Zijlstra
2013-11-06 14:44 ` Peter Zijlstra
2013-11-06 16:07 ` Peter Zijlstra
2013-11-06 17:31 ` Vince Weaver
2013-11-06 18:24 ` Peter Zijlstra
2013-11-07 8:21 ` Ingo Molnar
2013-11-07 14:27 ` Vince Weaver
2013-11-07 15:55 ` Ingo Molnar
2013-11-11 16:24 ` Peter Zijlstra
2013-11-11 21:10 ` Ingo Molnar
2013-10-29 21:23 ` perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-30 9:27 ` Paul E. McKenney
2013-10-30 11:25 ` Peter Zijlstra
2013-10-30 14:52 ` Victor Kaplansky
2013-10-30 15:39 ` Peter Zijlstra
2013-10-30 17:14 ` Victor Kaplansky
2013-10-30 17:44 ` Peter Zijlstra
2013-10-31 6:16 ` Paul E. McKenney
2013-11-01 13:12 ` Victor Kaplansky
2013-11-02 16:36 ` Paul E. McKenney
2013-11-02 17:26 ` Paul E. McKenney
2013-10-31 6:40 ` Paul E. McKenney
2013-11-01 14:25 ` Victor Kaplansky
2013-11-02 17:28 ` Paul E. McKenney
2013-11-01 14:56 ` Peter Zijlstra
2013-11-02 17:32 ` Paul E. McKenney
2013-11-03 14:40 ` Paul E. McKenney
2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra
2013-11-03 18:08 ` Linus Torvalds
2013-11-03 20:01 ` Peter Zijlstra
2013-11-03 22:42 ` Paul E. McKenney
2013-11-03 23:34 ` Linus Torvalds
2013-11-04 10:51 ` Paul E. McKenney
2013-11-04 11:22 ` Peter Zijlstra
2013-11-04 16:27 ` Paul E. McKenney
2013-11-04 16:48 ` Peter Zijlstra
2013-11-04 19:11 ` Peter Zijlstra
2013-11-04 19:18 ` Peter Zijlstra
2013-11-04 20:54 ` Paul E. McKenney
2013-11-04 20:53 ` Paul E. McKenney
2013-11-05 14:05 ` Will Deacon
2013-11-05 14:49 ` Paul E. McKenney
2013-11-05 18:49 ` Peter Zijlstra
2013-11-06 11:00 ` Will Deacon
2013-11-06 12:39 ` Peter Zijlstra
2013-11-06 12:51 ` Geert Uytterhoeven
2013-11-06 13:57 ` Peter Zijlstra
2013-11-06 18:48 ` Paul E. McKenney
2013-11-06 19:42 ` Peter Zijlstra
2013-11-07 11:17 ` Will Deacon
2013-11-07 13:36 ` Peter Zijlstra
2013-11-07 23:50 ` Mathieu Desnoyers
2013-11-04 11:05 ` Will Deacon
2013-11-04 16:34 ` Paul E. McKenney
2013-11-03 20:59 ` Benjamin Herrenschmidt
2013-11-03 22:43 ` Paul E. McKenney
2013-11-03 17:07 ` perf events ring buffer memory barrier on powerpc Will Deacon
2013-11-03 22:47 ` Paul E. McKenney
2013-11-04 9:57 ` Will Deacon
2013-11-04 10:52 ` Paul E. McKenney
2013-11-01 16:11 ` Peter Zijlstra
2013-11-02 17:46 ` Paul E. McKenney
2013-11-01 16:18 ` Peter Zijlstra
2013-11-02 17:49 ` Paul E. McKenney
2013-10-30 13:28 ` Victor Kaplansky
2013-10-30 15:51 ` Peter Zijlstra
2013-10-30 18:29 ` Peter Zijlstra
2013-10-30 19:11 ` Peter Zijlstra
2013-10-31 4:33 ` Paul E. McKenney
2013-10-31 4:32 ` Paul E. McKenney
2013-10-31 9:04 ` Peter Zijlstra
2013-10-31 15:07 ` Paul E. McKenney
2013-10-31 15:19 ` Peter Zijlstra
2013-11-01 9:28 ` Paul E. McKenney
2013-11-01 10:30 ` Peter Zijlstra
2013-11-02 15:20 ` Paul E. McKenney
2013-11-04 9:07 ` Peter Zijlstra [this message]
2013-11-04 10:00 ` Paul E. McKenney
2013-10-31 9:59 ` Victor Kaplansky
2013-10-31 12:28 ` David Laight
2013-10-31 12:55 ` Victor Kaplansky
2013-10-31 15:25 ` Paul E. McKenney
2013-11-01 16:06 ` Victor Kaplansky
2013-11-01 16:25 ` David Laight
2013-11-01 16:30 ` Victor Kaplansky
2013-11-03 20:57 ` Benjamin Herrenschmidt
2013-11-02 15:46 ` Paul E. McKenney
2013-10-28 19:09 ` Oleg Nesterov
2013-10-29 14:06 ` [tip:perf/urgent] perf: Fix perf ring buffer memory ordering tip-bot for Peter Zijlstra
2014-05-08 20:46 perf events ring buffer memory barrier on powerpc Mikulas Patocka
[not found] ` <OF667059AA.7F151BCC-ONC2257CD3.0036CFEB-C2257CD3.003BBF01@il.ibm.com>
2014-05-09 12:20 ` Mikulas Patocka
2014-05-09 13:47 ` Paul E. McKenney
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