[tip:sched/core] stop_machine: Clean up the usage of the preemption counter in cpu_stopper_thread()

From: tip-bot for Oleg Nesterov
Date: Mon Nov 23 2015 - 11:24:33 EST

Commit-ID: accaf6ea3db6f5fb997f096b6eefd5431d03f7e5
Gitweb: http://git.kernel.org/tip/accaf6ea3db6f5fb997f096b6eefd5431d03f7e5
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate: Sun, 15 Nov 2015 20:33:32 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 23 Nov 2015 09:48:20 +0100

stop_machine: Clean up the usage of the preemption counter in cpu_stopper_thread()

1. Change this code to use preempt_count_inc/preempt_count_dec; this way
it works even if CONFIG_PREEMPT_COUNT=n, and we avoid the unnecessary
__preempt_schedule() check (stop_sched_class is not preemptible).

And this makes clear that we only want to make preempt_count() != 0
for __might_sleep() / schedule_debug().

2. Change WARN_ONCE() to use %pf to print the function name and remove

3. Move "int ret" into the "if (work)" block, this looks more consistent.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Milos Vyletel <milos@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20151115193332.GA8281@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/stop_machine.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 7ff7ace..6110119 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -435,7 +435,6 @@ static void cpu_stopper_thread(unsigned int cpu)
struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
struct cpu_stop_work *work;
- int ret;

work = NULL;
@@ -451,23 +450,19 @@ repeat:
cpu_stop_fn_t fn = work->fn;
void *arg = work->arg;
struct cpu_stop_done *done = work->done;
- char ksym_buf[KSYM_NAME_LEN] __maybe_unused;
+ int ret;

- /* cpu stop callbacks are not allowed to sleep */
- preempt_disable();
+ /* cpu stop callbacks must not sleep, make in_atomic() == T */
+ preempt_count_inc();
ret = fn(arg);
if (done) {
if (ret)
done->ret = ret;
- /* restore preemption and check it's still balanced */
- preempt_enable();
+ preempt_count_dec();
- "cpu_stop: %s(%p) leaked preempt count\n",
- kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
- ksym_buf), arg);
+ "cpu_stop: %pf(%p) leaked preempt count\n", fn, arg);
goto repeat;
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