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[PATCH 8/8] stop_machine: cleanup the usage of preemption counter in cpu_stopper_thread()

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  [PATCH 8/8] stop_machine: cleanup the usage of preemption counter  in cpu_stopper_thread() Oleg Nesterov <oleg@redhat.com> - 2015-11-15 19:40 +0100
    Re: [PATCH 8/8] stop_machine: cleanup the usage of preemption  counter in cpu_stopper_thread() Tejun Heo <tj@kernel.org> - 2015-11-16 20:20 +0100
    [tip:sched/core] stop_machine:   Clean up the usage of the preemption counter in cpu_stopper_thread() tip-bot for Oleg Nesterov <tipbot@zytor.com> - 2015-11-23 17:30 +0100

#1269739 — [PATCH 8/8] stop_machine: cleanup the usage of 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
   kallsyms_lookup/ksym_buf.

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

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/stop_machine.c |   15 +++++----------
 1 files 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;
 
 repeat:
 	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;
 			cpu_stop_signal_done(done);
 		}
-		/* restore preemption and check it's still balanced */
-		preempt_enable();
+		preempt_count_dec();
 		WARN_ONCE(preempt_count(),
-			  "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;
 	}
 }
-- 
1.5.5.1

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#1270501 — Re: [PATCH 8/8] stop_machine: cleanup the usage of preemption counter in cpu_stopper_thread()

On Sun, Nov 15, 2015 at 08:33:32PM +0100, Oleg Nesterov wrote:
> 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
>    kallsyms_lookup/ksym_buf.
> 
> 3. Move "int ret" into the "if (work)" block, this looks more consistent.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

The entire series looks good to me.  Please feel free to add

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun
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#1275569 — [tip:sched/core] stop_machine: Clean up the usage of the preemption counter in cpu_stopper_thread()

Commit-ID:  accaf6ea3db6f5fb997f096b6eefd5431d03f7e5
Gitweb:     http://git.kernel.org/tip/accaf6ea3db6f5fb997f096b6eefd5431d03f7e5
Author:     Oleg Nesterov <oleg@redhat.com>
AuthorDate: Sun, 15 Nov 2015 20:33:32 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
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
   kallsyms_lookup/ksym_buf.

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

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Milos Vyletel <milos@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151115193332.GA8281@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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;
 
 repeat:
 	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;
 			cpu_stop_signal_done(done);
 		}
-		/* restore preemption and check it's still balanced */
-		preempt_enable();
+		preempt_count_dec();
 		WARN_ONCE(preempt_count(),
-			  "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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