The patch below, against 2.6.14-rc1, fixes a race condition wherein a CPU can
set itself in the nohz_cpu_mask just after a new grace period is started. If
the CPU continues to skip ticks after that, it can cause the grace
period that was started to be extended unnecessarily.
Signed-off-by : Srivatsa Vaddagiri <[email protected]>
---
linux-2.6.14-rc1-root/kernel/rcupdate.c | 34 ++++++++++++++++++++++++++++++--
1 files changed, 32 insertions(+), 2 deletions(-)
diff -puN kernel/rcupdate.c~rcu_race_fix kernel/rcupdate.c
--- linux-2.6.14-rc1/kernel/rcupdate.c~rcu_race_fix 2005-09-13 18:17:32.000000000 +0530
+++ linux-2.6.14-rc1-root/kernel/rcupdate.c 2005-09-13 18:17:42.000000000 +0530
@@ -86,6 +86,30 @@ spinlock_t __rcuref_hash[RCUREF_HASH_SIZ
};
#endif
+#ifdef CONFIG_NO_IDLE_HZ
+static inline int batch_complete(struct rcu_state *rsp)
+{
+ int rc = 0;
+ cpumask_t tmpmask;
+
+ cpus_andnot(tmpmask, rsp->cpumask, nohz_cpu_mask);
+ if (cpus_empty(tmpmask))
+ rc = 1;
+
+ return rc;
+}
+#else
+static inline int batch_complete(struct rcu_state *rsp)
+{
+ int rc = 0;
+
+ if (cpus_empty(rsp->cpumask))
+ rc = 1;
+
+ return rc;
+}
+#endif
+
/**
* call_rcu - Queue an RCU callback for invocation after a grace period.
* @head: structure to be used for queueing the RCU updates.
@@ -197,7 +221,13 @@ static void rcu_start_batch(struct rcu_c
if (rcp->next_pending &&
rcp->completed == rcp->cur) {
- /* Can't change, since spin lock held. */
+ /* cpu_online_map can't change, since a spinlock is held.
+ * However nohz_cpu_mask can change as we read it. A bad
+ * case of this race is when a CPU sets itself in the
+ * nohz_cpu_mask just after we have read it. We take care of
+ * this bad case when other CPUs call batch_complete() to check
+ * for batch completion.
+ */
cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);
rcp->next_pending = 0;
@@ -217,7 +247,7 @@ static void rcu_start_batch(struct rcu_c
static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp, struct rcu_state *rsp)
{
cpu_clear(cpu, rsp->cpumask);
- if (cpus_empty(rsp->cpumask)) {
+ if (batch_complete(rsp)) {
/* batch completed ! */
rcp->completed = rcp->cur;
rcu_start_batch(rcp, rsp, 0);
_
--
Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017