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

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Date:   Tue, 19 Nov 2019 08:50:11 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        will@...nel.org
Cc:     oleg@...hat.com, 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 11/13/19 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 */
> +}
> +
> +static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
> +{
> +	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
> +	bool wait;
> +
> +	spin_lock_irq(&sem->waiters.lock);
> +	/*
> +	 * Serialize against the wakeup in percpu_up_write(), if we fail
> +	 * the trylock, the wakeup must see us on the list.
> +	 */
> +	wait = !__percpu_rwsem_trylock(sem, reader);
> +	if (wait) {
> +		wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
> +		__add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
> +	}
> +	spin_unlock_irq(&sem->waiters.lock);
> +
> +	while (wait) {
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +		if (!smp_load_acquire(&wq_entry.private))
> +			break;
> +		schedule();
> +	}

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.
However, the comment above said we wake 'all' readers and 1 writer. That
doesn't match the actual code, IMO. 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.

Cheers,
Longman