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Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org,
        will@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org,
        bigeasy@linutronix.de, juri.lelli@redhat.com, williams@redhat.com,
        bristot@redhat.com, dave@stgolabs.net, jack@suse.com
References: <20191113102115.116470462@infradead.org>
 <20191113102855.925208237@infradead.org>
 <ee75fc38-c3c8-3f9e-13ba-5c8312d61325@redhat.com>
 <20191119155826.GA4739@redhat.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <efc4ba6b-b113-b37f-c785-b03df1afc860@redhat.com>
Date:   Tue, 19 Nov 2019 11:28:50 -0500
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On 11/19/19 10:58 AM, Oleg Nesterov wrote:
> On 11/19, Waiman Long wrote:
>> On 11/13/19 5:21 AM, Peter Zijlstra wrote:
>>> +static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entr=
y,
>>> +=09=09=09=09      unsigned int mode, int wake_flags,
>>> +=09=09=09=09      void *key)
>>> +{
>>> +=09struct task_struct *p =3D get_task_struct(wq_entry->private);
>>> +=09bool reader =3D wq_entry->flags & WQ_FLAG_CUSTOM;
>>> +=09struct percpu_rw_semaphore *sem =3D key;
>>> +
>>> +=09/* concurrent against percpu_down_write(), can get stolen */
>>> +=09if (!__percpu_rwsem_trylock(sem, reader))
>>> +=09=09return 1;
>>> +
>>> +=09list_del_init(&wq_entry->entry);
>>> +=09smp_store_release(&wq_entry->private, NULL);
>>> +
>>> +=09wake_up_process(p);
>>> +=09put_task_struct(p);
>>> +
>>> +=09return !reader; /* wake 'all' readers and 1 writer */
>>> +}
>>> +
>> If I read the function correctly, you are setting the WQ_FLAG_EXCLUSIVE
>> for both readers and writers and __wake_up() is called with an exclusive
>> count of one. So only one reader or writer is woken up each time.
> This depends on what percpu_rwsem_wake_function() returns. If it returns =
1,
> __wake_up_common() stops, exactly because all waiters have WQ_FLAG_EXCLUS=
IVE.
>
>> However, the comment above said we wake 'all' readers and 1 writer. That
>> doesn't match the actual code, IMO.
> Well, "'all' readers" probably means "all readers before writer",
>
>> To match the comments, you should
>> have set WQ_FLAG_EXCLUSIVE flag only on writer. In this case, you
>> probably don't need WQ_FLAG_CUSTOM to differentiate between readers and
>> writers.
> See above...
>
> note also the
>
> =09if (!__percpu_rwsem_trylock(sem, reader))
> =09=09return 1;
>
> at the start of percpu_rwsem_wake_function(). We want to stop wake_up_com=
mon()
> as soon as percpu_rwsem_trylock() fails. Because we know that if it fails=
 once
> it can't succeed later. Although iiuc this can only happen if another (ne=
w)
> writer races with __wake_up(&sem->waiters).
>
>
> I guess WQ_FLAG_CUSTOM can be avoided, percpu_rwsem_wait() could do
>
> =09if (read)
> =09=09__add_wait_queue_entry_tail(...);
> =09else {
> =09=09wq_entry.flags |=3D WQ_FLAG_EXCLUSIVE;
> =09=09__add_wait_queue(...);
> =09}
>
> but this is "unfair".

Thanks for the explanation. That clarifies my understanding of the patch.

Cheers,
Longman