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-rw-r--r--kernel/rcutree_plugin.h18
1 files changed, 16 insertions, 2 deletions
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 643c8f650dd0..c22c4ef2a0d0 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1041,6 +1041,8 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
* robustness against future implementation changes.
*/
smp_mb(); /* See above comment block. */
+ if (cpumask_first(cpu_online_mask) == smp_processor_id())
+ atomic_inc(&synchronize_sched_expedited_count);
return 0;
}
@@ -1053,13 +1055,26 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
* Note that it is illegal to call this function while holding any
* lock that is acquired by a CPU-hotplug notifier. Failing to
* observe this restriction will result in deadlock.
+ *
+ * The synchronize_sched_expedited_cpu_stop() function is called
+ * in stop-CPU context, but in order to keep overhead down to a dull
+ * roar, we don't force this function to wait for its counterparts
+ * on other CPUs. One instance of this function will increment the
+ * synchronize_sched_expedited_count variable per call to
+ * try_stop_cpus(), but there is no guarantee what order this instance
+ * will occur in. The worst case is that it is last on one call
+ * to try_stop_cpus(), and the first on the next call. This means
+ * that piggybacking requires that synchronize_sched_expedited_count
+ * be incremented by 3: this guarantees that the piggybacking
+ * task has waited through an entire cycle of context switches,
+ * even in the worst case.
*/
void synchronize_sched_expedited(void)
{
int snap, trycount = 0;
smp_mb(); /* ensure prior mod happens before capturing snap. */
- snap = atomic_read(&synchronize_sched_expedited_count) + 1;
+ snap = atomic_read(&synchronize_sched_expedited_count) + 2;
get_online_cpus();
while (try_stop_cpus(cpu_online_mask,
synchronize_sched_expedited_cpu_stop,
@@ -1077,7 +1092,6 @@ void synchronize_sched_expedited(void)
}
get_online_cpus();
}
- atomic_inc(&synchronize_sched_expedited_count);
smp_mb__after_atomic_inc(); /* ensure post-GP actions seen after GP. */
put_online_cpus();
}