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

[Oleg Nesterov <oleg@redhat.com>]

Paul, thanks for looking!

On 10/15, Paul E. McKenney wrote:
>
> > +void brw_start_read(struct brw_mutex *brw)
> > +{
> > +	for (;;) {
> > +		bool done = false;
> > +
> > +		preempt_disable();
> > +		if (likely(!atomic_read(&brw->write_ctr))) {
> > +			__this_cpu_inc(*brw->read_ctr);
> > +			done = true;
> > +		}
>
> brw_start_read() is not recursive -- attempting to call it recursively
> can result in deadlock if a writer has shown up in the meantime.

Yes, yes, it is not recursive. Like rw_semaphore.

> Which is often OK, but not sure what you intended.

I forgot to document this in the changelog.

> > +void brw_end_read(struct brw_mutex *brw)
> > +{
>
> I believe that you need smp_mb() here.

I don't understand why...

> The wake_up_all()'s memory barriers
> do not suffice because some other reader might have awakened the writer
> between this_cpu_dec() and wake_up_all().

But __wake_up(q) takes q->lock? And the same lock is taken by
prepare_to_wait(), so how can the writer miss the result of _dec?

> > +	this_cpu_dec(*brw->read_ctr);
> > +
> > +	if (unlikely(atomic_read(&brw->write_ctr)))
> > +		wake_up_all(&brw->write_waitq);
> > +}
>
> Of course, it would be good to avoid smp_mb on the fast path.  Here is
> one way to avoid it:
>
> void brw_end_read(struct brw_mutex *brw)
> {
> 	if (unlikely(atomic_read(&brw->write_ctr))) {
> 		smp_mb();
> 		this_cpu_dec(*brw->read_ctr);
> 		wake_up_all(&brw->write_waitq);

Hmm... still can't understand.

It seems that this mb() is needed to ensure that brw_end_read() can't
miss write_ctr != 0.

But we do not care unless the writer already does wait_event(). And
before it does wait_event() it calls synchronize_sched() after it sets
write_ctr != 0. Doesn't this mean that after that any preempt-disabled
section must see write_ctr != 0 ?

This code actually checks write_ctr after preempt_disable + enable,
but I think this doesn't matter?

Paul, most probably I misunderstood you. Could you spell please?

> > +void brw_start_write(struct brw_mutex *brw)
> > +{
> > +	atomic_inc(&brw->write_ctr);
> > +	synchronize_sched();
> > +	/*
> > +	 * Thereafter brw_*_read() must see write_ctr != 0,
> > +	 * and we should see the result of __this_cpu_inc().
> > +	 */
> > +	wait_event(brw->write_waitq, brw_read_ctr(brw) == 0);
>
> This looks like it allows multiple writers to proceed concurrently.
> They both increment, do a synchronize_sched(), do the wait_event(),
> and then are both awakened by the last reader.

Yes. From the changelog:

	Unlike rw_semaphore it allows multiple writers too,
	just "read" and "write" are mutually exclusive.

> Was that the intent?  (The implementation of brw_end_write() makes
> it look like it is in fact the intent.)

Please look at 2/2.

Multiple uprobe_register() or uprobe_unregister() can run at the
same time to install/remove the system-wide breakpoint, and
brw_start_write() is used to block dup_mmap() to avoid the race.
But they do not block each other.

Thanks!

Oleg.