4.2-stable review patch. If anyone has any objections, please let me know.
------------------
From: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
commit 00cc1633816de8c95f337608a1ea64e228faf771 upstream.
Commit 2ee507c47293 ("sched: Add function single_task_running to let a task
check if it is the only task running on a cpu") referenced the current
runqueue with the smp_processor_id. When CONFIG_DEBUG_PREEMPT is enabled,
that is only allowed if preemption is disabled or the currrent task is
bound to the local cpu (e.g. kernel worker).
With commit f78195129963 ("kvm: add halt_poll_ns module parameter") KVM
calls single_task_running. If CONFIG_DEBUG_PREEMPT is enabled that
generates a lot of kernel messages.
To avoid adding preemption in that cases, as it would limit the usefulness,
we change single_task_running to access directly the cpu local runqueue.
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Fixes: 2ee507c472939db4b146d545352b8a7c79ef47f8
Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/core.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2614,13 +2614,20 @@ unsigned long nr_running(void)
/*
* Check if only the current task is running on the cpu.
+ *
+ * Caution: this function does not check that the caller has disabled
+ * preemption, thus the result might have a time-of-check-to-time-of-use
+ * race. The caller is responsible to use it correctly, for example:
+ *
+ * - from a non-preemptable section (of course)
+ *
+ * - from a thread that is bound to a single CPU
+ *
+ * - in a loop with very short iterations (e.g. a polling loop)
*/
bool single_task_running(void)
{
- if (cpu_rq(smp_processor_id())->nr_running == 1)
- return true;
- else
- return false;
+ return raw_rq()->nr_running == 1;
}
EXPORT_SYMBOL(single_task_running);
--
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