Commit 618758ed authored by Mathieu Desnoyers's avatar Mathieu Desnoyers Committed by Peter Zijlstra
Browse files

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: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201020134715.13909-3-mathieu.desnoyers@efficios.com
parent 5bc78502

kernel/kthread.c

+21 −0
Original line number Diff line number Diff line
@@ -1248,6 +1248,7 @@ void kthread_use_mm(struct mm_struct *mm) 
		tsk->active_mm = mm;

	}

	tsk->mm = mm;

	membarrier_update_current_mm(mm);

	switch_mm_irqs_off(active_mm, mm, tsk);

	local_irq_enable();

	task_unlock(tsk);
@@ -1255,8 +1256,19 @@ void kthread_use_mm(struct mm_struct *mm) 
	finish_arch_post_lock_switch();

#endif



	/*

	 * When a kthread starts operating on an address space, the loop

	 * in membarrier_{private,global}_expedited() may not observe

	 * that tsk->mm, and not issue an IPI. Membarrier requires a

	 * memory barrier after storing to tsk->mm, before accessing

	 * user-space memory. A full memory barrier for membarrier

	 * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by

	 * mmdrop(), or explicitly with smp_mb().

	 */

	if (active_mm != mm)

		mmdrop(active_mm);

	else

		smp_mb();



	to_kthread(tsk)->oldfs = force_uaccess_begin();

}
@@ -1276,9 +1288,18 @@ void kthread_unuse_mm(struct mm_struct *mm) 
	force_uaccess_end(to_kthread(tsk)->oldfs);



	task_lock(tsk);

	/*

	 * When a kthread stops operating on an address space, the loop

	 * in membarrier_{private,global}_expedited() may not observe

	 * that tsk->mm, and not issue an IPI. Membarrier requires a

	 * memory barrier after accessing user-space memory, before

	 * clearing tsk->mm.

	 */

	smp_mb__after_spinlock();

	sync_mm_rss(mm);

	local_irq_disable();

	tsk->mm = NULL;

	membarrier_update_current_mm(NULL);

	/* active_mm is still 'mm' */

	enter_lazy_tlb(mm, tsk);

	local_irq_enable();

kernel/sched/idle.c

+1 −0
Original line number Diff line number Diff line
@@ -338,6 +338,7 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns) 
	WARN_ON_ONCE(!(current->flags & PF_KTHREAD));

	WARN_ON_ONCE(!(current->flags & PF_NO_SETAFFINITY));

	WARN_ON_ONCE(!duration_ns);

	WARN_ON_ONCE(current->mm);



	rcu_sleep_check();

	preempt_disable();

kernel/sched/membarrier.c

+3 −4
Original line number Diff line number Diff line
@@ -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);