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rcu: Eliminate __rcu_pending() false positives

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Now that there are both ->gpnum and ->completed fields in the
rcu_node structure, __rcu_pending() should check rdp->gpnum and
rdp->completed against rnp->gpnum and rdp->completed, respectively,
instead of the prior comparison against the rcu_state fields
rsp->gpnum and rsp->completed.

Given the old comparison, __rcu_pending() could return 1, resulting
in a needless raise_softirq(RCU_SOFTIRQ).  This useless work would
happen if RCU responded to a scheduling-clock interrupt after the
rcu_state fields had been updated, but before the rcu_node fields
had been updated.

Changing the comparison from the rcu_state fields to the rcu_node
fields prevents this useless work from happening.

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: [email protected]
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LKML-Reference: <12581706991966-git-send-email->
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Paul E. McKenney 2009-11-13 19:51:39 -08:00 • committed by Ingo Molnar
commit 2f51f9884f
1 changed files with 4 additions and 2 deletions
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6
kernel/rcutree.c
View file

@ -1403,6 +1403,8 @@ EXPORT_SYMBOL_GPL(call_rcu_bh);
*/
static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
{
struct rcu_node *rnp = rdp->mynode;
rdp->n_rcu_pending++;
/* Check for CPU stalls, if enabled. */
@ -1427,13 +1429,13 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
}
/* Has another RCU grace period completed? */
if (ACCESS_ONCE(rsp->completed) != rdp->completed) { /* outside lock */
if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
rdp->n_rp_gp_completed++;
return 1;
}
/* Has a new RCU grace period started? */
if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) { /* outside lock */
if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */
rdp->n_rp_gp_started++;
return 1;
}