Re: [patch 00/15] Generic Mutex Subsystem

[Oleg Nesterov <oleg@tv-sign.ru>]

Linus Torvalds wrote:
> 
> > > [ Oh.  I'm looking at the semaphore code, and I realize that we have a
> > >   "wake_up(&sem->wait)" in the __down() path because we had some race long
> > >   ago that we fixed by band-aiding over it. Which means that we wake up
> > >   sleepers that shouldn't be woken up. THAT may well be part of the
> > >   performance problem.. The semaphores are really meant to wake up just
> > >   one at a time, but because of that race hack they'll wake up _two_ at a
> > >   time - once by up(), once by down().
> > >
> > >   That also destroys the fairness. Does anybody remember why it's that
> > >   way? ]
> >
> > History?
> 
> Oh, absolutely, I already checked the old BK history too, and that extra
> wake_up() has been there at least since before we even started using BK.
> So it's very much historical, I'm just wondering if somebody remembers far
> enough back that we'd know.
> 
> I don't see why it's needed (since we re-try the "atomic_add_negative()"
> inside the semaphore wait lock, and any up() that saw contention should
> have always been guaranteed to do a wakeup that should fill the race in
> between that atomic_add_negative() and the thing going to sleep).
> 
> It may be that it is _purely_ historical, and simply isn't needed. That
> would be funny/sad, in the sense that we've had it there for years and
> years ;)

This does not look like it was added to fix a race or historical
to me. I think without that "wake_up" a waiter can miss wakeup
if it is not the only one sleeper.

sem->count == 0, ->sleepers == 0. down() decrements ->count,

__down:
	// ->count == -1

	++sleepers; // == 1

	for (;;) {
		->count += (->sleepers - 1); // does nothing
		if (->count >= 0) // NO
			break;

		->sleepers = 1;
		schedule();
	...

Another process calls down(), ->count == -2

__down:
	++sleepers; // == 2;

	for (;;) {
		->count += (->sleepers - 1) // ->count == -1;

		->sleepers = 1;
		schedule();
	...

up() makes ++->count == 0, and wakes one of them. It will see
->sleepers == 1, so atomic_add_negative(sleepers - 1) again
has no effect, sets ->sleepers == 0 and takes the semaphore.

Note that subsequent up() will not call wakeup(): ->count == 0,
it just increment it. That is why we are waking the next waiter
in advance. When it gets cpu, it will decrement ->count by 1,
because ->sleepers == 0. If up() (++->count) was already called,
it takes semaphore. If not - goes to sleep again.

Or my understanding is completely broken?

Oleg.
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