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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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]
This commit is contained in:
Paul E. McKenney 2013-07-26 20:47:42 -07:00
parent 5361471437
commit 458fb381ea
1 changed files with 13 additions and 2 deletions
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15
kernel/rcutree.c
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@ -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);