From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, Dave Chinner <david@fromorbit.com>,
Eric Sandeen <sandeen@redhat.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers
Date: Tue, 24 Apr 2018 10:49:08 -0400 [thread overview]
Message-ID: <801fea56-90f3-4965-b24b-fa88ac8d16ce@redhat.com> (raw)
In-Reply-To: <20180424091510.GB4064@hirez.programming.kicks-ass.net>
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
prev parent reply other threads:[~2018-04-24 14:49 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-10 17:22 [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers Waiman Long
2018-04-18 6:20 ` Benjamin Herrenschmidt
2018-04-23 16:46 ` Waiman Long
2018-04-23 20:55 ` Andrea Parri
2018-04-23 21:30 ` Waiman Long
2018-04-24 9:15 ` Peter Zijlstra
2018-04-24 14:49 ` Waiman Long [this message]
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