sys_membarrier() scheduler barrier requirements: dropped

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello!

Commit 3ed668659e95 ("membarrier: Document scheduler barrier
requirements") did not make it into the v4.14 merge window, and rebasing
to v4.14-rc1 results in conflicts.  I have therefore dropped it.
If someone would be willing to forward-port it, I would be quite happy
to pull it back in for the v4.15 merge window.  Or potentially as part
of the fix to sys_membarrier() for 4.14, for that matter.

							Thanx, Paul

------------------------------------------------------------------------

commit 3ed668659e95ecfb6f6be0a3e7ff0fa6d27b2f5c
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date:   Fri Aug 18 21:39:16 2017 -0700

    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__before_spinlock() in __schedule().
    
    Document that membarrier requires a full barrier on transition from
    kernel thread to userspace thread, which skips the call to switch_mm(). 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 load_cr3(). Document that load_cr3() is providing
    this barrier.
    
    [ Rebased on top of linux-rcu for-mingo branch.
      Applies on top of "membarrier: Provide expedited private command". ]
    
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    CC: Peter Zijlstra <peterz@infradead.org>
    CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    CC: Boqun Feng <boqun.feng@gmail.com>
    CC: Andrew Hunter <ahh@google.com>
    CC: Maged Michael <maged.michael@gmail.com>
    CC: gromer@google.com
    CC: Avi Kivity <avi@scylladb.com>
    CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    CC: Paul Mackerras <paulus@samba.org>
    CC: Michael Ellerman <mpe@ellerman.id.au>
    CC: Dave Watson <davejwatson@fb.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 014d07a80053..cd815b63420a 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 	 * and neither LOCK nor MFENCE orders them.
 	 * Fortunately, load_cr3() is serializing and gives the
 	 * ordering guarantee we need.
+	 *
+	 * This full barrier is also required by the membarrier
+	 * system call.
 	 */
 	load_cr3(next->pgd);
 
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2b24a6974847..fe29d06e2800 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,10 @@ 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.
+	 */
 	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
 		__mmdrop(mm);
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3f29c6a89d80..b0f199f9ec62 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2654,6 +2654,12 @@ 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)) {
@@ -3295,6 +3301,9 @@ 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.
 	 */
 	smp_mb__before_spinlock();
 	rq_lock(rq, &rf);