Re: [PATCH 1/2] brw_mutex: big read-write mutex

[Oleg Nesterov <oleg@redhat.com>]

On 10/19, Mikulas Patocka wrote:
>
> synchronize_rcu() is way slower than msleep(1) -

This depends, I guess. but this doesn't mmatter,

> so I don't see a reason
> why should it be complicated to avoid msleep(1).

I don't think this really needs complications. Please look at this
patch for example. Or initial (single writer) version below. It is
not finished and lacks the barriers too, but I do not think it is
more complex.

Oleg.

struct brw_sem {
	long __percpu		*read_ctr;
	wait_queue_head_t	read_waitq;
	struct mutex		writer_mutex;
	struct task_struct	*writer;
};

int brw_init(struct brw_sem *brw)
{
	brw->writer = NULL;
	mutex_init(&brw->writer_mutex);
	init_waitqueue_head(&brw->read_waitq);
	brw->read_ctr = alloc_percpu(long);
	return brw->read_ctr ? 0 : -ENOMEM;
}

void brw_down_read(struct brw_sem *brw)
{
	for (;;) {
		bool done = false;

		preempt_disable();
		if (likely(!brw->writer)) {
			__this_cpu_inc(*brw->read_ctr);
			done = true;
		}
		preempt_enable();

		if (likely(done))
			break;

		__wait_event(brw->read_waitq, !brw->writer);
	}
}

void brw_up_read(struct brw_sem *brw)
{
	struct task_struct *writer;

	preempt_disable();
	__this_cpu_dec(*brw->read_ctr);
	writer = ACCESS_ONCE(brw->writer);
	if (unlikely(writer))
		wake_up_process(writer);
	preempt_enable();
}

static inline long brw_read_ctr(struct brw_sem *brw)
{
	long sum = 0;
	int cpu;

	for_each_possible_cpu(cpu)
		sum += per_cpu(*brw->read_ctr, cpu);

	return sum;
}

void brw_down_write(struct brw_sem *brw)
{
	mutex_lock(&brw->writer_mutex);
	brw->writer = current;
	synchronize_sched();
	/*
	 * Thereafter brw_*_read() must see ->writer != NULL,
	 * and we should see the result of __this_cpu_inc().
	 */
	for (;;) {
		set_current_state(TASK_UNINTERRUPTIBLE);
		if (brw_read_ctr(brw) == 0)
			break;
		schedule();
	}
	__set_current_state(TASK_RUNNING);
	/*
	 * We can add another synchronize_sched() to avoid the
	 * spurious wakeups from brw_up_read() after return.
	 */
}

void brw_up_write(struct brw_sem *brw)
{
	brw->writer = NULL;
	synchronize_sched();
	wake_up_all(&brw->read_waitq);
	mutex_unlock(&brw->writer_mutex);
}