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SubjectRe: [PATCH 4/7] preempt: Disable preemption from preempt_schedule*() callers
On Mon, May 11, 2015 at 05:08:21PM +0200, Frederic Weisbecker wrote:
> Lets gather the preempt operations (set PREEMPT_ACTIVE and disable
> preemption) in a single operation. This way we prepare to remove the
> preemption disablement in __schedule() in order to omptimize this
> duty on the caller.
>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
> include/linux/preempt.h | 12 ++++++++++++
> kernel/sched/core.c | 20 ++++++--------------
> 2 files changed, 18 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 4689ef2..45da394 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -137,6 +137,18 @@ extern void preempt_count_sub(int val);
> #define preempt_count_inc() preempt_count_add(1)
> #define preempt_count_dec() preempt_count_sub(1)
>
> +#define preempt_active_enter() \
> +do { \
> + preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
> + barrier(); \
> +} while (0)
> +
> +#define preempt_active_exit() \
> +do { \
> + barrier(); \
> + preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
> +} while (0)
> +
> #ifdef CONFIG_PREEMPT_COUNT
>
> #define preempt_disable() \
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8027cfd..182127a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2859,15 +2859,14 @@ void __sched schedule_preempt_disabled(void)
> static void __sched notrace preempt_schedule_common(void)
> {
> do {
> - __preempt_count_add(PREEMPT_ACTIVE);
> + preempt_active_enter();
> __schedule();
> - __preempt_count_sub(PREEMPT_ACTIVE);
> + preempt_active_exit();
>
> /*
> * Check again in case we missed a preemption opportunity
> * between schedule and now.
> */
> - barrier();
> } while (need_resched());
> }

So this patch adds a level of preempt_disable; I suspect the goal is to
remove the preempt_disable() inside __schedule(), but as it stands this
patch is broken, no?


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