[PATCH v3 tip/core/rcu 4/4] srcu: Reduce probability of SRCU ->unlock_count[] counter overflow

[Paul E. McKenney]

Because there are no memory barriers between the srcu_flip() ->completed
increment and the summation of the read-side ->unlock_count[] counters,
both the compiler and the CPU can reorder the summation with the
->completed increment.  If the updater is preempted long enough during
this process, the read-side counters could overflow, resulting in a
too-short grace period.

This commit therefore adds a memory barrier just after the ->completed
increment, ensuring that if the summation misses an increment of
->unlock_count[] from __srcu_read_unlock(), the next __srcu_read_lock()
will see the new value of ->completed, thus bounding the number of
->unlock_count[] increments that can be missed to NR_CPUS.  The actual
overflow computation is more complex due to the possibility of nesting
of __srcu_read_lock().

Reported-by: Lance Roy <ldr709@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/srcu.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index fcd07eda95a3..5dbd0d4426ff 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -321,7 +321,16 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
  */
 static void srcu_flip(struct srcu_struct *sp)
 {
-	sp->completed++;
+	WRITE_ONCE(sp->completed, sp->completed + 1);
+
+	/*
+	 * Ensure that if the updater misses an __srcu_read_unlock()
+	 * increment, that task's next __srcu_read_lock() will see the
+	 * above counter update.  Note that both this memory barrier
+	 * and the one in srcu_readers_active_idx_check() provide the
+	 * guarantee for __srcu_read_lock().
+	 */
+	smp_mb(); /* D */  /* Pairs with C. */
 }
 
 /*
-- 
2.5.2