linux-kernel - Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem

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Date:   Tue, 19 Nov 2019 11:28:50 -0500
From:   Waiman Long <longman@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        will@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, juri.lelli@...hat.com, williams@...hat.com,
        bristot@...hat.com, dave@...olabs.net, jack@...e.com
Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem

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_entry,
>>> +				      unsigned int mode, int wake_flags,
>>> +				      void *key)
>>> +{
>>> +	struct task_struct *p = get_task_struct(wq_entry->private);
>>> +	bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
>>> +	struct percpu_rw_semaphore *sem = key;
>>> +
>>> +	/* concurrent against percpu_down_write(), can get stolen */
>>> +	if (!__percpu_rwsem_trylock(sem, reader))
>>> +		return 1;
>>> +
>>> +	list_del_init(&wq_entry->entry);
>>> +	smp_store_release(&wq_entry->private, NULL);
>>> +
>>> +	wake_up_process(p);
>>> +	put_task_struct(p);
>>> +
>>> +	return !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_EXCLUSIVE.
>
>> 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
>
> 	if (!__percpu_rwsem_trylock(sem, reader))
> 		return 1;
>
> at the start of percpu_rwsem_wake_function(). We want to stop wake_up_common()
> 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 (new)
> writer races with __wake_up(&sem->waiters).
>
>
> I guess WQ_FLAG_CUSTOM can be avoided, percpu_rwsem_wait() could do
>
> 	if (read)
> 		__add_wait_queue_entry_tail(...);
> 	else {
> 		wq_entry.flags |= WQ_FLAG_EXCLUSIVE;
> 		__add_wait_queue(...);
> 	}
>
> but this is "unfair".

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

Cheers,
Longman