linux-kernel - Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

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Date:   Thu, 29 Nov 2018 17:06:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Yongji Xie <elohimes@...il.com>, mingo@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        xieyongji@...du.com, zhangyu31@...du.com, liuqi16@...du.com,
        yuanlinsi01@...du.com, nixun@...du.com, lilin24@...du.com,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the
 reader waiter to nil

On Thu, Nov 29, 2018 at 10:21:58AM -0500, Waiman Long wrote:

> >> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> >> index 09b1800..50d9af6 100644
> >> --- a/kernel/locking/rwsem-xadd.c
> >> +++ b/kernel/locking/rwsem-xadd.c
> >> @@ -198,15 +198,22 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> >>  		woken++;
> >>  		tsk = waiter->task;
> >>  
> >> -		wake_q_add(wake_q, tsk);
> >> +		get_task_struct(tsk);
> >>  		list_del(&waiter->list);
> >>  		/*
> >> -		 * Ensure that the last operation is setting the reader
> >> +		 * Ensure calling get_task_struct() before setting the reader
> >>  		 * waiter to nil such that rwsem_down_read_failed() cannot
> >>  		 * race with do_exit() by always holding a reference count
> >>  		 * to the task to wakeup.
> >>  		 */
> >>  		smp_store_release(&waiter->task, NULL);
> >> +		/*
> >> +		 * Ensure issuing the wakeup (either by us or someone else)
> >> +		 * after setting the reader waiter to nil.
> >> +		 */
> >> +		wake_q_add(wake_q, tsk);
> >> +		/* wake_q_add() already take the task ref */
> >> +		put_task_struct(tsk);
> >>  	}
> >>  
> >>  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
> 
> I doubt putting wake_q_add() after clearing waiter->task can really fix

Why; at that point we know the wakeup will happen after, which is all we
require.