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

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Message-ID: <20191119155826.GA4739@redhat.com>
Date:   Tue, 19 Nov 2019 16:58:26 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Waiman Long <longman@...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, 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".

Oleg.