[PATCH 04/10] rcu/nocb: Remove needless full barrier after callback advancing

[Frederic Weisbecker <frederic@xxxxxxxxxx>]

A full barrier is issued from nocb_gp_wait() upon callbacks advancing
to order grace period completion with callbacks execution.

However these two events are already ordered by the
smp_mb__after_unlock_lock() barrier within the call to
raw_spin_lock_rcu_node() that is necessary for callbacks advancing to
happen.

The following litmus test shows the kind of guarantee that this barrier
provides:

	C smp_mb__after_unlock_lock

	{}

	// rcu_gp_cleanup()
	P0(spinlock_t *rnp_lock, int *gpnum)
	{
		// Grace period cleanup increase gp sequence number
		spin_lock(rnp_lock);
		WRITE_ONCE(*gpnum, 1);
		spin_unlock(rnp_lock);
	}

	// nocb_gp_wait()
	P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready)
	{
		int r1;

		// Call rcu_advance_cbs() from nocb_gp_wait()
		spin_lock(nocb_lock);
		spin_lock(rnp_lock);
		smp_mb__after_unlock_lock();
		r1 = READ_ONCE(*gpnum);
		WRITE_ONCE(*cb_ready, 1);
		spin_unlock(rnp_lock);
		spin_unlock(nocb_lock);
	}

	// nocb_cb_wait()
	P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed)
	{
		int r2;

		// rcu_do_batch() -> rcu_segcblist_extract_done_cbs()
		spin_lock(nocb_lock);
		r2 = READ_ONCE(*cb_ready);
		spin_unlock(nocb_lock);

		// Actual callback execution
		WRITE_ONCE(*cb_executed, 1);
	}

	P3(int *cb_executed, int *gpnum)
	{
		int r3;

		WRITE_ONCE(*cb_executed, 2);
		smp_mb();
		r3 = READ_ONCE(*gpnum);
	}

	exists (1:r1=1 /\ 2:r2=1 /\ cb_executed=2 /\ 3:r3=0) (* Bad outcome. *)

Here the bad outcome only occurs if the smp_mb__after_unlock_lock() is
removed. This barrier orders the grace period completion against
callbacks advancing and even later callbacks invocation, thanks to the
opportunistic propagation via the ->nocb_lock to nocb_cb_wait().

Therefore the smp_mb() placed after callbacks advancing can be safely
removed.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/rcu/tree_nocb.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 6e63ba4788e1..2dc76f5e6e78 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -779,7 +779,6 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
 			needwake = rdp->nocb_cb_sleep;
 			WRITE_ONCE(rdp->nocb_cb_sleep, false);
-			smp_mb(); /* CB invocation -after- GP end. */
 		} else {
 			needwake = false;
 		}
-- 
2.41.0