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(-)
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
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