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: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter 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: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/tlb.c

@@ -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

@@ -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

@@ -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;