Re: [RFC][PATCH 00/13] percpu rwsem -v2

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sun, Jul 19, 2015 at 10:53 PM, Daniel Wagner <wagi@monom.org> wrote:
>
> Turning them back into arch_spinlock_t gives almost the same numbers as
> with spinlock_t.
>
> Then Peter suggested to change the code to
>
>         preempt_disable();
>         spin_unlock();
>         preempt_enable_no_resched();
>
> to verify if arch_spin_lock() is buggy and does not disable preemption
> and we see a lock holder preemption on non virt setups.

Hmm. "arch_spin_lock()" isn't _supposed_ to disable preemption. The
caller should do that (possibly by disabling interrupts). See
include/linux/spinlock_api_smp.h for details.

But yes, that's a *very* subtle difference between "arch_spin_lock()"
and "spin_lock()". The former doesn't do lockdep or other debugging
and it doesn't disable preemption. So they are not interchangeable.

The current lglocks uses arch_spin_lock exactly because it does not
*want* lockdep tracking (it does its own) and because it does its own
preemption handling.

So saying "verify if arch_spin_lock() is buggy and does not disable
preemption" is complete BS. If arch_spin_lock() were to disable
preemption, _that_ would be a bug.

                Linus