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Commit 458fb381 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Simplify _rcu_barrier() processing 

This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work
is done" check in _rcu_barrier().  This applies feedback from Linus
(https://lkml.org/lkml/2013/7/26/777

) that he gave to similar code
in an unrelated patch.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]
parent 53614714

kernel/rcutree.c

+13 −2
Original line number Diff line number Diff line
@@ -2817,9 +2817,20 @@ static void _rcu_barrier(struct rcu_state *rsp) 
	 * transition.  The "if" expression below therefore rounds the old

	 * value up to the next even number and adds two before comparing.

	 */

	snap_done = ACCESS_ONCE(rsp->n_barrier_done);

	snap_done = rsp->n_barrier_done;

	_rcu_barrier_trace(rsp, "Check", -1, snap_done);

	if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {



	/*

	 * If the value in snap is odd, we needed to wait for the current

	 * rcu_barrier() to complete, then wait for the next one, in other

	 * words, we need the value of snap_done to be three larger than

	 * the value of snap.  On the other hand, if the value in snap is

	 * even, we only had to wait for the next rcu_barrier() to complete,

	 * in other words, we need the value of snap_done to be only two

	 * greater than the value of snap.  The "(snap + 3) & ~0x1" computes

	 * this for us (thank you, Linus!).

	 */

	if (ULONG_CMP_GE(snap_done, (snap + 3) & ~0x1)) {

		_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);

		smp_mb(); /* caller's subsequent code after above check. */

		mutex_unlock(&rsp->barrier_mutex);