linux-kernel - Re: [RFC PATCH 01/11] locking/ww_mutex: Remove wakeups from under mutex::wait

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Date:   Tue, 4 Oct 2022 12:01:13 -0400
From:   Waiman Long <longman@...hat.com>
To:     Connor O'Brien <connoro@...gle.com>, linux-kernel@...r.kernel.org
Cc:     kernel-team@...roid.com, John Stultz <jstultz@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Qais Yousef <qais.yousef@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [RFC PATCH 01/11] locking/ww_mutex: Remove wakeups from under
 mutex::wait_lock

On 10/3/22 17:44, Connor O'Brien wrote:
> diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
> index 56f139201f24..dfc174cd96c6 100644
> --- a/kernel/locking/ww_mutex.h
> +++ b/kernel/locking/ww_mutex.h
> @@ -161,6 +161,11 @@ static inline void lockdep_assert_wait_lock_held(struct rt_mutex *lock)
>   
>   #endif /* WW_RT */
>   
> +void ww_ctx_wake(struct ww_acquire_ctx *ww_ctx)
> +{
> +	wake_up_q(&ww_ctx->wake_q);
> +}
> +
>   /*
>    * Wait-Die:
>    *   The newer transactions are killed when:
> @@ -284,7 +289,7 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
>   #ifndef WW_RT
>   		debug_mutex_wake_waiter(lock, waiter);
>   #endif
> -		wake_up_process(waiter->task);
> +		wake_q_add(&ww_ctx->wake_q, waiter->task);
>   	}
>   
>   	return true;
> @@ -331,7 +336,7 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
>   		 * wakeup pending to re-read the wounded state.
>   		 */
>   		if (owner != current)
> -			wake_up_process(owner);
> +			wake_q_add(&ww_ctx->wake_q, owner);
>   
>   		return true;
>   	}
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ee28253c9ac0..617e737392be 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1013,6 +1013,13 @@ void wake_up_q(struct wake_q_head *head)
>   		wake_up_process(task);
>   		put_task_struct(task);
>   	}
> +	/*
> +	 * XXX connoro: seems this is needed now that ww_ctx_wake() passes in a
> +	 * wake_q_head that is embedded in struct ww_acquire_ctx rather than
> +	 * declared locally.
> +	 */
> +	head->first = node;
> +	head->lastp = &head->first;
>   }
>   

You shouldn't do wake_q_init() here in wake_up_q(). Instead, you should 
do it in ww_ctx_wake() right after the wake_up_q() call.

Cheers,
Longman