[ Added Peter ]
On Thu, 5 Jul 2018 17:50:34 +0200
Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> migrate_disable() does nothing !SMP && !RT. This is bad for two reasons:
> - The futex code relies on the fact migrate_disable() is part of spin_lock().
> There is a workaround for the !in_atomic() case in migrate_disable() which
> work-arounds the different ordering (non-atomic lock and atomic unlock).
But isn't it only part of spin_lock() in the RT case?
>
> - we have a few instances where preempt_disable() is replaced with
> migrate_disable().
What? Really? I thought we only replace preempt_disable() with a
local_lock(). Which gives annotation to why a preempt_disable() exists.
And on non-RT, local_lock() is preempt_disable().
>
> For both cases it is bad if migrate_disable() ends up as barrier() instead of
> preempt_disable(). Let migrate_disable() fallback to preempt_disable().
>
I still don't understand exactly what is "bad" about it.
IIRC, I remember Peter not wanting any open coded "migrate_disable"
calls. It was to be for internal use cases only, and specifically, only
for RT.
Personally, I think making migrate_disable() into preempt_disable() on
NON_RT is incorrect too.
-- Steve
> Cc: stable-rt@xxxxxxxxxxxxxxx
> Reported-by: joe.korty@xxxxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> include/linux/preempt.h | 4 ++--
> kernel/sched/core.c | 2 ++
> 2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 043e431a7e8e..d46688d521e6 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -241,8 +241,8 @@ static inline int __migrate_disabled(struct task_struct *p)
> }
>
> #else
> -#define migrate_disable() barrier()
> -#define migrate_enable() barrier()
> +#define migrate_disable() preempt_disable()
> +#define migrate_enable() preempt_enable()
> static inline int __migrate_disabled(struct task_struct *p)
> {
> return 0;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ac3fb8495bd5..626a62218518 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7326,6 +7326,7 @@ void migrate_disable(void)
> #endif
>
> p->migrate_disable++;
> + preempt_disable();
> }
> EXPORT_SYMBOL(migrate_disable);
>
> @@ -7349,6 +7350,7 @@ void migrate_enable(void)
>
> WARN_ON_ONCE(p->migrate_disable <= 0);
> p->migrate_disable--;
> + preempt_enable();
> }
> EXPORT_SYMBOL(migrate_enable);
> #endif
--
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