Re: [PATCH v2 06/12] sched/wait: Add a waitqueue helper for fully exclusive priority waiters

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 19, 2025 at 11:55:08AM -0700, Sean Christopherson wrote:
> Add a waitqueue helper to add a priority waiter that requires exclusive
> wakeups, i.e. that requires that it be the _only_ priority waiter.  The
> API will be used by KVM to ensure that at most one of KVM's irqfds is
> bound to a single eventfd (across the entire kernel).
> 
> Open code the helper instead of using __add_wait_queue() so that the
> common path doesn't need to "handle" impossible failures.
> 
> Note, the priority_exclusive() name is obviously confusing as the plain
> priority() API also sets WQ_FLAG_EXCLUSIVE.  This will be remedied once
> KVM switches to add_wait_queue_priority_exclusive(), as the only other
> user of add_wait_queue_priority(), Xen's privcmd, doesn't actually operate
> in exclusive mode (more than likely, the detail was overlooked when privcmd
> copy-pasted (sorry, "was inspired by") KVM's implementation).
> 
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  include/linux/wait.h |  2 ++
>  kernel/sched/wait.c  | 18 ++++++++++++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 965a19809c7e..09855d819418 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -164,6 +164,8 @@ static inline bool wq_has_sleeper(struct wait_queue_head *wq_head)
>  extern void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);
>  extern void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);
>  extern void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);
> +extern int add_wait_queue_priority_exclusive(struct wait_queue_head *wq_head,
> +					     struct wait_queue_entry *wq_entry);
>  extern void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);
>  
>  static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> index 51e38f5f4701..03252badb8e8 100644
> --- a/kernel/sched/wait.c
> +++ b/kernel/sched/wait.c
> @@ -47,6 +47,24 @@ void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_
>  }
>  EXPORT_SYMBOL_GPL(add_wait_queue_priority);
>  
> +int add_wait_queue_priority_exclusive(struct wait_queue_head *wq_head,
> +				      struct wait_queue_entry *wq_entry)
> +{
> +	struct list_head *head = &wq_head->head;
> +
> +	wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY;
> +
> +	guard(spinlock_irqsave)(&wq_head->lock);
> +
> +	if (!list_empty(head) &&
> +	    (list_first_entry(head, typeof(*wq_entry), entry)->flags & WQ_FLAG_PRIORITY))
> +		return -EBUSY;
> +
> +	list_add(&wq_entry->entry, head);
> +	return 0;
> +}
> +EXPORT_SYMBOL(add_wait_queue_priority_exclusive);

add_wait_queue_priority() is a GPL export, leading me to believe the
whole priority thing is _GPL only, should we maintain that?