linux-kernel - Re: [PATCH tip/core/rcu 1/4] sched,rcu: Make cond

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Message-ID: <20171005104051.shazuwnqbqjj3be2@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Oct 2017 12:40:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 1/4] sched,rcu: Make cond_resched() provide
 RCU quiescent state

On Wed, Oct 04, 2017 at 02:50:22PM -0700, Paul E. McKenney wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 26a7df4e558c..59688ef3ea23 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1565,10 +1565,11 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
>   * cond_resched_lock() will drop the spinlock before scheduling,
>   * cond_resched_softirq() will enable bhs before scheduling.
>   */
> +void rcu_all_qs(void);
>  #ifndef CONFIG_PREEMPT
>  extern int _cond_resched(void);
>  #else
> -static inline int _cond_resched(void) { return 0; }
> +static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
>  #endif

Why does it need to change PREEMPT? the whole cond_resched() thing
should be a no-op on PREEMPT.