Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative

[Linus Torvalds]

On Tue, Apr 23, 2019 at 7:17 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> I'm not aware of an architecture where disabling interrupts is faster
> than disabling preemption.

I don't thin kit ever is, but I'd worry a bit about the
preempt_enable() just because it also checks if need_resched() is true
when re-enabling preemption.

So doing preempt_enable() as part of rwsem_read_trylock() might cause
us to schedule in *exactly* the wrong place,

So if we play preemption games, I wonder if we should make them more
explicit than hiding them in that helper function, because
particularly for the slow path case, I think we'd be much better off
just avoiding the busy-loop in the slow path, rather than first
scheduling due to preempt_enable(), and then starting to look at the
slow path onlly afterwards.

IOW, I get the feeling that the preemption-off area might be better
off being potentially much bigger, and covering the whole (or a large
portion) of the semaphore operation, rather than just the
rwsem_read_trylock() fastpath.

Hmm?

                   Linus