linux-kernel - Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock

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Date:   Thu, 17 Aug 2017 10:26:16 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace
 spin_unlock_wait() with lock/unlock pair


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> There is no agreed-upon definition of spin_unlock_wait()'s semantics,
> and it appears that all callers could do just as well with a lock/unlock
> pair.  This commit therefore replaces the spin_unlock_wait() call in
> completion_done() with spin_lock() followed immediately by spin_unlock().
> This should be safe from a performance perspective because the lock
> will be held only the wakeup happens really quickly.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Andrea Parri <parri.andrea@...il.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> [ paulmck: Updated to use irqsave based on 0day Test Robot feedback. ]
> 
> diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> index 13fc5ae9bf2f..c9524d2d9316 100644
> --- a/kernel/sched/completion.c
> +++ b/kernel/sched/completion.c
> @@ -300,6 +300,8 @@ EXPORT_SYMBOL(try_wait_for_completion);
>   */
>  bool completion_done(struct completion *x)
>  {
> +	unsigned long flags;
> +
>  	if (!READ_ONCE(x->done))
>  		return false;
>  
> @@ -307,14 +309,9 @@ bool completion_done(struct completion *x)
>  	 * If ->done, we need to wait for complete() to release ->wait.lock
>  	 * otherwise we can end up freeing the completion before complete()
>  	 * is done referencing it.
> -	 *
> -	 * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
> -	 * the loads of ->done and ->wait.lock such that we cannot observe
> -	 * the lock before complete() acquires it while observing the ->done
> -	 * after it's acquired the lock.
>  	 */
> -	smp_rmb();
> -	spin_unlock_wait(&x->wait.lock);
> +	spin_lock_irqsave(&x->wait.lock, flags);
> +	spin_unlock_irqrestore(&x->wait.lock, flags);
>  	return true;
>  }
>  EXPORT_SYMBOL(completion_done);

I'm fine with this patch - as long as there are no performance regression reports. 
(which I suspect there won't be.)

Would you like to carry this in the RCU tree, due to other changes depending on 
this change - or can I pick this up into the scheduler tree?

Thanks,

	Ingo