On Tue, Oct 27, 2020 at 10:56:36AM -0700, Sean Christopherson wrote:
> On Tue, Oct 27, 2020 at 09:49:50AM -0700, Ben Gardon wrote:
> > Rescheduling while holding a spin lock is essential for keeping long
> > running kernel operations running smoothly. Add the facility to
> > cond_resched rwlocks.
>
> This adds two new exports and two new macros without any in-tree users, which
> is generally frowned upon. You and I know these will be used by KVM's new
> TDP MMU, but the non-KVM folks, and more importantly the maintainers of this
> code, are undoubtedly going to ask "why". I.e. these patches probably belong
> in the KVM series to switch to a rwlock for the TDP MMU.
I was informed about this ;-)
> Regarding the code, it's all copy-pasted from the spinlock code and darn near
> identical. It might be worth adding builder macros for these.
I considered mentioning them; I'm typically a fan of them, but I'm not
quite sure it's worth the effort here.
> > +int __cond_resched_rwlock_read(rwlock_t *lock)
> > +{
> > + int resched = should_resched(PREEMPT_LOCK_OFFSET);
> > + int ret = 0;
> > +
> > + lockdep_assert_held(lock);
> > +
> > + if (rwlock_needbreak(lock) || resched) {
> > + read_unlock(lock);
> > + if (resched)
> > + preempt_schedule_common();
> > + else
> > + cpu_relax();
> > + ret = 1;
>
> AFAICT, this rather odd code flow from __cond_resched_lock() is an artifact of
> code changes over the years and not intentionally weird. IMO, it would be
> cleaner and easier to read as:
>
> int resched = should_resched(PREEMPT_LOCK_OFFSET);
>
> lockdep_assert_held(lock);
lockdep_assert_held_read() :-)
>
> if (!rwlock_needbreak(lock) && !resched)
> return 0;
>
> read_unlock(lock);
> if (resched)
> preempt_schedule_common();
> else
> cpu_relax();
> read_lock(lock)
> return 1;
>
I suppose that works, but then also change the existing one.
