On Thu, Jul 27, 2017 at 02:59:43PM -0400, Mathieu Desnoyers wrote:
> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> index 9f9284f37f8d..8c6c0f96f617 100644
> --- a/kernel/membarrier.c
> +++ b/kernel/membarrier.c
> @@ -19,10 +19,81 @@
> #include <linux/tick.h>
>
> /*
> + * XXX For cpu_rq(). Should we rather move
> + * membarrier_private_expedited() to sched/core.c or create
> + * sched/membarrier.c ?
The later perhaps.
> +static void membarrier_private_expedited(void)
> +{
> + int cpu, this_cpu;
> + cpumask_var_t tmpmask;
> +
> + if (num_online_cpus() == 1)
> + return;
> +
> + /*
> + * Matches memory barriers around rq->curr modification in
> + * scheduler.
> + */
> + smp_mb(); /* system call entry is not a mb. */
> +
> + if (!alloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
Why GFP_NOWAIT ? and falback. There seems to be a desire to make this a
nonblocking syscall. Should we document this somewhere?
> + /* Fallback for OOM. */
> + membarrier_private_expedited_ipi_each();
> + goto end;
> + }
> +
> + this_cpu = raw_smp_processor_id();
This is a tad dodgy, you might want to put in a comment on how migrating
this thread is ok.
> + for_each_online_cpu(cpu) {
One would also need cpus_read_lock() if you rely on the online mask.
> + struct task_struct *p;
> +
> + if (cpu == this_cpu)
> + continue;
> + rcu_read_lock();
> + p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> + if (p && p->mm == current->mm)
> + __cpumask_set_cpu(cpu, tmpmask);
> + rcu_read_unlock();
> + }
> + smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> + free_cpumask_var(tmpmask);
> +end:
> + /*
> + * Memory barrier on the caller thread _after_ we finished
> + * waiting for the last IPI. Matches memory barriers around
> + * rq->curr modification in scheduler.
> + */
> + smp_mb(); /* exit from system call is not a mb */
> +}
> @@ -2737,6 +2757,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
>
> mm = next->mm;
> oldmm = prev->active_mm;
> + membarrier_expedited_mb_after_set_current(mm, oldmm);
> /*
> * For paravirt, this is coupled with an exit in switch_to to
> * combine the page table reload and the switch backend into
As said on IRC, we have finish_task_switch()->if (mm)
mmdrop(mm)->atomic_dec_and_test() providing a smp_mb(). We just need to
deal with the !mm case.
