sched: membarrier: cover kthread_use_mm (v4)
Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm
to allow the effect of membarrier(2) to apply to kthreads accessing
user-space memory as well.
Given that no prior kthread use this guarantee and that it only affects
kthreads, adding this guarantee does not affect user-space ABI.
Refine the check in membarrier_global_expedited to exclude runqueues
running the idle thread rather than all kthreads from the IPI cpumask.
Now that membarrier_global_expedited can IPI kthreads, the scheduler
also needs to update the runqueue's membarrier_state when entering lazy
TLB state.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201020134715.13909-3-mathieu.desnoyers@efficios.com
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index aac3292..f223f35 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -126,12 +126,11 @@ static int membarrier_global_expedited(void)
continue;
/*
- * Skip the CPU if it runs a kernel thread. The scheduler
- * leaves the prior task mm in place as an optimization when
- * scheduling a kthread.
+ * Skip the CPU if it runs a kernel thread which is not using
+ * a task mm.
*/
p = rcu_dereference(cpu_rq(cpu)->curr);
- if (p->flags & PF_KTHREAD)
+ if (!p->mm)
continue;
__cpumask_set_cpu(cpu, tmpmask);