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Fix RCU race in access of nohz_cpu_mask

PATCH In review Srivatsa Vaddagiri 2 participants 2 messages Dec 5, 2005 – Dec 8, 2005

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Srivatsa Vaddagiri

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#0 lore raw

PATCH 1/1 Fix RCU race in access of nohz_cpu_mask

Accessing nohz_cpu_mask before incrementing rcp->cur is racy. It can cause tickless idle CPUs to be included in rsp->cpumask, which will extend graceperiods unnecessarily.

Patch below (against 2.6.15-rc5-mm1) fixes this race. It has been tested using extensions to RCU torture module that forces various CPUs to become idle.

Signed-off-by Srivatsa Vaddagiri <[email protected]>

---

 linux-2.6.15-rc5-mm1-root/kernel/rcupdate.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
kernel/rcupdate.c renamed from kernel/rcupdate.c~rcu-lock +3 -3
diff -puN kernel/rcupdate.c~rcu-lock kernel/rcupdate.c
@@ -244,15 +244,15 @@ static void rcu_start_batch(struct rcu_c
 
 	if (rcp->next_pending &&
 			rcp->completed == rcp->cur) {
-		/* Can't change, since spin lock held. */
-		cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);
-
 		rcp->next_pending = 0;
 		/* next_pending == 0 must be visible in __rcu_process_callbacks()
 		 * before it can see new value of cur.
 		 */
 		smp_wmb();
 		rcp->cur++;
+		smp_mb();
+		cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);
+
 	}
 }

_

Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017

#1 lore raw

kernel/rcupdate.c static void rcu_start_batch(struct rcu_c · line 254

Quoted 3 lines · Srivatsa Vaddagiri
kernel/rcupdate.c renamed from kernel/rcupdate.c~rcu-lock +2 -0
+		smp_mb();
+		cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);

Please always include a comment when adding a barrier. Because it's often not obvious to the reader what that barrier is actually doing.