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Message-ID: <1262852862.4049.78.camel@laptop>
Date: Thu, 07 Jan 2010 09:27:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc: linux-kernel@...r.kernel.org,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
Valdis.Kletnieks@...edu, dhowells@...hat.com, laijs@...fujitsu.com,
dipankar@...ibm.com, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
barrier
On Wed, 2010-01-06 at 23:40 -0500, Mathieu Desnoyers wrote:
> Index: linux-2.6-lttng/kernel/sched.c
> ===================================================================
> --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-06 22:11:32.000000000 -0500
> +++ linux-2.6-lttng/kernel/sched.c 2010-01-06 23:20:42.000000000 -0500
> @@ -10822,6 +10822,36 @@ struct cgroup_subsys cpuacct_subsys = {
> };
> #endif /* CONFIG_CGROUP_CPUACCT */
>
> +/*
> + * Execute a memory barrier on all CPUs on SMP systems.
> + * Do not rely on implicit barriers in smp_call_function(), just in case they
> + * are ever relaxed in the future.
> + */
> +static void membarrier_ipi(void *unused)
> +{
> + smp_mb();
> +}
> +
> +/*
> + * sys_membarrier - issue memory barrier on current process running threads
> + *
> + * Execute a memory barrier on all running threads of the current process.
> + * Upon completion, the caller thread is ensured that all process threads
> + * have passed through a state where memory accesses match program order.
> + * (non-running threads are de facto in such a state)
> + *
> + * The current implementation simply executes a memory barrier in an IPI handler
> + * on each active cpu. Going through the hassle of taking run queue locks and
> + * checking if the thread running on each online CPU belongs to the current
> + * thread seems more heavyweight than the cost of the IPI itself.
> + */
> +SYSCALL_DEFINE0(membarrier)
> +{
> + on_each_cpu(membarrier_ipi, NULL, 1);
> +
> + return 0;
> +}
> +
> #ifndef CONFIG_SMP
>
> int rcu_expedited_torture_stats(char *page)
OK, so my worry here is that its a DoS on large machines.
Something like:
smp_call_function_any(current->mm->cpu_vm_mask, membarrier, NULL, 1);
might be slightly better, but would still hurt. The alternative is
iterating all cpus and looking to see if cpu_curr(cpu)->mm ==
current->mm and then send it an ipi.
Also, there was some talk a while ago about IPIs implying memory
barriers. Which I of course forgot all details about,.. at least sending
one implies a wmb and receiving one an rmb, but it could be stronger,
Oleg?
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