[tip:sched/urgent] Revert "sched/core: Do not use smp_processor_id() with preempt enabled in smpboot_thread_fn()"

From: tip-bot for Ingo Molnar
Date: Tue Oct 04 2016 - 04:09:14 EST

Commit-ID: be6a2e4c46cc122ba9113ba569fbc50fad075fff
Gitweb: http://git.kernel.org/tip/be6a2e4c46cc122ba9113ba569fbc50fad075fff
Author: Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate: Tue, 4 Oct 2016 09:55:57 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 4 Oct 2016 09:55:57 +0200

Revert "sched/core: Do not use smp_processor_id() with preempt enabled in smpboot_thread_fn()"

This reverts commit 4fa5cd5245b627db88c9ca08ae442373b02596b4.

The original change widens a preempt-off section, to avoid a seemingly unsafe
smp_processor_id() use.

During review I overlooked two facts:

- The code to calls a non-trivial function callback:


... which might (and does occasionally) sleep, triggering the warning.

- More importantly, as pointed out by Peter Zijlstra, using
smp_processor_id() in that context is safe, if it's done from
a kernel thread that is pinned to a single CPU - which is the
case here.

So revert to the original code that enables preemption sooner.

Reported-by: kernel test robot <xiaolong.ye@xxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Con Kolivas <kernel@xxxxxxxxxxx>
Cc: Alfred Chen <cchalpha@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20160930015102.GB20189@yexl-desktop
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/smpboot.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index fc0d8270..13bc43d 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -122,12 +122,12 @@ static int smpboot_thread_fn(void *data)

if (kthread_should_park()) {
+ preempt_enable();
if (ht->park && td->status == HP_THREAD_ACTIVE) {
BUG_ON(td->cpu != smp_processor_id());
td->status = HP_THREAD_PARKED;
- preempt_enable();
/* We might have been woken for stop */