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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. ]
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@ -2817,9 +2817,20 @@ static void _rcu_barrier(struct rcu_state *rsp)
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* transition. The "if" expression below therefore rounds the old
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* value up to the next even number and adds two before comparing.
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*/
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snap_done = ACCESS_ONCE(rsp->n_barrier_done);
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snap_done = rsp->n_barrier_done;
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_rcu_barrier_trace(rsp, "Check", -1, snap_done);
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if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
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/*
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* If the value in snap is odd, we needed to wait for the current
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* rcu_barrier() to complete, then wait for the next one, in other
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* words, we need the value of snap_done to be three larger than
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* the value of snap. On the other hand, if the value in snap is
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* even, we only had to wait for the next rcu_barrier() to complete,
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* in other words, we need the value of snap_done to be only two
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* greater than the value of snap. The "(snap + 3) & ~0x1" computes
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* this for us (thank you, Linus!).
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*/
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if (ULONG_CMP_GE(snap_done, (snap + 3) & ~0x1)) {
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_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
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smp_mb(); /* caller's subsequent code after above check. */
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mutex_unlock(&rsp->barrier_mutex);
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