[PATCH v2 08/12] sched/wait: Drop WQ_FLAG_EXCLUSIVE from add_wait_queue_priority()

[Sean Christopherson]

On Tue, May 20, 2025, Peter Zijlstra wrote:
> On Mon, May 19, 2025 at 11:55:10AM -0700, Sean Christopherson wrote:
> > Drop the setting of WQ_FLAG_EXCLUSIVE from add_wait_queue_priority() to
> > differentiate it from add_wait_queue_priority_exclusive().  The one and
> > only user add_wait_queue_priority(), Xen privcmd's irqfd_wakeup(),
> > unconditionally returns '0', i.e. doesn't actually operate in exclusive
> > mode.
> 
> I find:
> 
> drivers/hv/mshv_eventfd.c:      add_wait_queue_priority(wqh, &irqfd->irqfd_wait);
> drivers/xen/privcmd.c:  add_wait_queue_priority(wqh, &kirqfd->wait);
> 
> I mean, it might still be true and all, but hyperv seems to also use
> this now.

Oh FFS, another "heavily inspired by KVM".  I should have bribed someone to take
this series when I had the chance.  *sigh*

Unfortunately, the Hyper-V code does actually operate in exclusive mode.  Unless
you have a better idea, I'll tweak the series to:

  1. Drop WQ_FLAG_EXCLUSIVE from add_wait_queue_priority() and have the callers
     explicitly set the flag, 
  2. Add a patch to drop WQ_FLAG_EXCLUSIVE from Xen privcmd entirely.
  3. Introduce add_wait_queue_priority_exclusive() and switch KVM to use it.

That has an added bonus of introducing the Xen change in a dedicated patch, i.e.
is probably a sequence anyways.

Alternatively, I could rewrite the Hyper-V code a la the KVM changes, but I'm not
feeling very charitable at the moment (the complete lack of documentation for
their ioctl doesn't help).