linux-kernel - Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers

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Date:   Tue, 24 Apr 2018 10:49:08 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Dave Chinner <david@...morbit.com>,
        Eric Sandeen <sandeen@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of
 readers

On 04/24/2018 05:15 AM, Peter Zijlstra wrote:
> On Mon, Apr 23, 2018 at 10:55:14PM +0200, Andrea Parri wrote:
>>>> +	/*
>>>> +	 * To avoid missed wakeup of reader, we need to make sure
>>>> +	 * that task state and waiter->task are properly synchronized.
>>>> +	 *
>>>> +	 *     wakeup		      sleep
>>>> +	 *     ------		      -----
>>>> +	 * __rwsem_mark_wake:	rwsem_down_read_failed*:
>>>> +	 *   [S] waiter->task	  [S] set_current_state(state)
>>>> +	 *	 MB		      MB
>>>> +	 * try_to_wake_up:
>>>> +	 *   [L] state		  [L] waiter->task
>>>> +	 *
>>>> +	 * For the wakeup path, the original lock release-acquire pair
>>>> +	 * does not provide enough guarantee of proper synchronization.
>>>> +	 */
>>>> +	smp_mb();
>>>> +
>>>>  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
>>>>  	if (list_empty(&sem->wait_list)) {
>>>>  		/* hit end of list above */
>> try_to_wake_up() does:
>>
>> 	raw_spin_lock_irqsave(&p->pi_lock, flags);
>> 	smp_mb__after_spinlock();
>> 	if (!(p->state & state))
>>
>> My understanding is that this smp_mb__after_spinlock() provides us with
>> the guarantee you described above.  The smp_mb__after_spinlock() should
>> represent a 'cheaper way' to provide such a guarantee.
> Right, I don't see what problem is being fixed here either. The scenario
> in the comment is already closed by the smp_mb__after_spinlock() you
> mention.
>
> And it is fine to rely on that, we do in other places.

Right, I missed the smp_mb__after_spinlock(). So the upstream code is
fine after all. Sorry for the noise.

Cheers,
Longman