[PATCH] Fix RCU race in access of nohz_cpu_mask

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

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 <vatsa@in.ibm.com>

---

 linux-2.6.15-rc5-mm1-root/kernel/rcupdate.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff -puN kernel/rcupdate.c~rcu-lock kernel/rcupdate.c
--- linux-2.6.15-rc5-mm1/kernel/rcupdate.c~rcu-lock	2005-12-05 15:28:33.000000000 +0530
+++ linux-2.6.15-rc5-mm1-root/kernel/rcupdate.c	2005-12-05 15:28:40.000000000 +0530
@@ -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