From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: linux-kernel@vger.kernel.org
Cc: mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com,
tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
Valdis.Kletnieks@vt.edu, dhowells@redhat.com,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/2] rcu: eliminate __rcu_pending() false positives
Date: Fri, 13 Nov 2009 19:51:39 -0800 [thread overview]
Message-ID: <12581706991966-git-send-email-> (raw)
In-Reply-To: <20091114035100.GA9599@linux.vnet.ibm.com>
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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 <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1089d08..e3565cd 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1390,6 +1390,8 @@ 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. */
@@ -1414,13 +1416,13 @@ 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;
}
--
1.5.2.5
next prev parent reply other threads:[~2009-11-14 3:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-14 3:51 [PATCH tip/core/rcu 0/2] rcu: lastcomp cleanups, fix rcu_pending() false positives Paul E. McKenney
2009-11-14 3:51 ` [PATCH tip/core/rcu 1/2] rcu: further cleanups of use of lastcomp Paul E. McKenney
2009-11-14 10:18 ` [tip:core/rcu] rcu: Further " tip-bot for Paul E. McKenney
2009-11-14 3:51 ` Paul E. McKenney [this message]
2009-11-14 10:19 ` [tip:core/rcu] rcu: Eliminate __rcu_pending() false positives tip-bot for Paul E. McKenney
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