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

From: Paul E. McKenney
Date: Tue Jan 24 2017 - 17:00:56 EST

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. */