Re: [PATCH 1/4] sched: Better document the try_to_wake_up() barriers

[Byungchul Park]

On Mon, Nov 02, 2015 at 02:29:02PM +0100, Peter Zijlstra wrote:
> Explain how the control dependency and smp_rmb() end up providing
> ACQUIRE semantics and pair with smp_store_release() in
> finish_lock_switch().
> 
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c  |    8 +++++++-
>  kernel/sched/sched.h |    3 +++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1947,7 +1947,13 @@ try_to_wake_up(struct task_struct *p, un
>  	while (p->on_cpu)
>  		cpu_relax();
>  	/*
> -	 * Pairs with the smp_wmb() in finish_lock_switch().
> +	 * Combined with the control dependency above, we have an effective
> +	 * smp_load_acquire() without the need for full barriers.
> +	 *
> +	 * Pairs with the smp_store_release() in finish_lock_switch().
> +	 *
> +	 * This ensures that tasks getting woken will be fully ordered against
> +	 * their previous state and preserve Program Order.
>  	 */
>  	smp_rmb();

Hello,

I am not sure, but just curious..

I thought it's good enough to use a kind of dependancy barrier such as
smp_read_barrier_depends() instead of smp_rmb() here unless something
e.g. compiler breaks the control dependacy. Right?

>  
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1073,6 +1073,9 @@ static inline void finish_lock_switch(st
>  	 * We must ensure this doesn't happen until the switch is completely
>  	 * finished.
>  	 *
> +	 * In particular, the load of prev->state in finish_task_switch() must
> +	 * happen before this.
> +	 *
>  	 * Pairs with the control dependency and rmb in try_to_wake_up().
>  	 */
>  	smp_store_release(&prev->on_cpu, 0);
> 
> 
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