Commit 306e0604 authored by Mathieu Desnoyers's avatar Mathieu Desnoyers Committed by Ingo Molnar
Browse files

membarrier: Document scheduler barrier requirements 



Document the membarrier requirement on having a full memory barrier in
__schedule() after coming from user-space, before storing to rq->curr.
It is provided by smp_mb__after_spinlock() in __schedule().

Document that membarrier requires a full barrier on transition from
kernel thread to userspace thread. We currently have an implicit barrier
from atomic_dec_and_test() in mmdrop() that ensures this.

The x86 switch_mm_irqs_off() full barrier is currently provided by many
cpumask update operations as well as write_cr3(). Document that
write_cr3() provides this barrier.

Signed-off-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Andrew Hunter <ahh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dave Watson <davejwatson@fb.com>
Cc: David Sehr <sehr@google.com>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maged Michael <maged.michael@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/20180129202020.8515-4-mathieu.desnoyers@efficios.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3ccfebed

arch/x86/mm/tlb.c

+5 −0
Original line number Diff line number Diff line
@@ -228,6 +228,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, 
#endif

	this_cpu_write(cpu_tlbstate.is_lazy, false);



	/*

	 * The membarrier system call requires a full memory barrier

	 * before returning to user-space, after storing to rq->curr.

	 * Writing to CR3 provides that full memory barrier.

	 */

	if (real_prev == next) {

		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=

			   next->context.ctx_id);

include/linux/sched/mm.h

+5 −0
Original line number Diff line number Diff line
@@ -39,6 +39,11 @@ static inline void mmgrab(struct mm_struct *mm) 
extern void __mmdrop(struct mm_struct *);

static inline void mmdrop(struct mm_struct *mm)

{

	/*

	 * The implicit full barrier implied by atomic_dec_and_test() is

	 * required by the membarrier system call before returning to

	 * user-space, after storing to rq->curr.

	 */

	if (unlikely(atomic_dec_and_test(&mm->mm_count)))

		__mmdrop(mm);

}

kernel/sched/core.c

+26 −11
Original line number Diff line number Diff line
@@ -2703,6 +2703,12 @@ static struct rq *finish_task_switch(struct task_struct *prev) 
	finish_arch_post_lock_switch();



	fire_sched_in_preempt_notifiers(current);

	/*

	 * When transitioning from a kernel thread to a userspace

	 * thread, mmdrop()'s implicit full barrier is required by the

	 * membarrier system call, because the current ->active_mm can

	 * become the current mm without going through switch_mm().

	 */

	if (mm)

		mmdrop(mm);

	if (unlikely(prev_state == TASK_DEAD)) {
@@ -2808,6 +2814,13 @@ context_switch(struct rq *rq, struct task_struct *prev, 
	 */

	arch_start_context_switch(prev);



	/*

	 * If mm is non-NULL, we pass through switch_mm(). If mm is

	 * NULL, we will pass through mmdrop() in finish_task_switch().

	 * Both of these contain the full memory barrier required by

	 * membarrier after storing to rq->curr, before returning to

	 * user-space.

	 */

	if (!mm) {

		next->active_mm = oldmm;

		mmgrab(oldmm);
@@ -3344,6 +3357,9 @@ static void __sched notrace __schedule(bool preempt) 
	 * Make sure that signal_pending_state()->signal_pending() below

	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)

	 * done by the caller to avoid the race with signal_wake_up().

	 *

	 * The membarrier system call requires a full memory barrier

	 * after coming from user-space, before storing to rq->curr.

	 */

	rq_lock(rq, &rf);

	smp_mb__after_spinlock();
@@ -3391,17 +3407,16 @@ static void __sched notrace __schedule(bool preempt) 
		/*

		 * The membarrier system call requires each architecture

		 * to have a full memory barrier after updating

		 * rq->curr, before returning to user-space. For TSO

		 * (e.g. x86), the architecture must provide its own

		 * barrier in switch_mm(). For weakly ordered machines

		 * for which spin_unlock() acts as a full memory

		 * barrier, finish_lock_switch() in common code takes

		 * care of this barrier. For weakly ordered machines for

		 * which spin_unlock() acts as a RELEASE barrier (only

		 * arm64 and PowerPC), arm64 has a full barrier in

		 * switch_to(), and PowerPC has

		 * smp_mb__after_unlock_lock() before

		 * finish_lock_switch().

		 * rq->curr, before returning to user-space.

		 *

		 * Here are the schemes providing that barrier on the

		 * various architectures:

		 * - mm ? switch_mm() : mmdrop() for x86, s390, sparc, PowerPC.

		 *   switch_mm() rely on membarrier_arch_switch_mm() on PowerPC.

		 * - finish_lock_switch() for weakly-ordered

		 *   architectures where spin_unlock is a full barrier,

		 * - switch_to() for arm64 (weakly-ordered, spin_unlock

		 *   is a RELEASE barrier),

		 */

		++*switch_count;

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