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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
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    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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