Re: [patch V2 11/15] completion: Use simple wait queues

[Davidlohr Bueso]

On Wed, 18 Mar 2020, Thomas Gleixner wrote:

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> completion uses a wait_queue_head_t to enqueue waiters.
>
> wait_queue_head_t contains a spinlock_t to protect the list of waiters
> which excludes it from being used in truly atomic context on a PREEMPT_RT
> enabled kernel.
>
> The spinlock in the wait queue head cannot be replaced by a raw_spinlock
> because:
>
>  - wait queues can have custom wakeup callbacks, which acquire other
>    spinlock_t locks and have potentially long execution times
>
>  - wake_up() walks an unbounded number of list entries during the wake up
>    and may wake an unbounded number of waiters.
>
> For simplicity and performance reasons complete() should be usable on
> PREEMPT_RT enabled kernels.
>
> completions do not use custom wakeup callbacks and are usually single
> waiter, except for a few corner cases.
>
> Replace the wait queue in the completion with a simple wait queue (swait),
> which uses a raw_spinlock_t for protecting the waiter list and therefore is
> safe to use inside truly atomic regions on PREEMPT_RT.
>
> There is no semantical or functional change:
>
>  - completions use the exclusive wait mode which is what swait provides
>
>  - complete() wakes one exclusive waiter
>
>  - complete_all() wakes all waiters while holding the lock which protects
>    the wait queue against newly incoming waiters. The conversion to swait
>    preserves this behaviour.
>
> complete_all() might cause unbound latencies with a large number of waiters
> being woken at once, but most complete_all() usage sites are either in
> testing or initialization code or have only a really small number of
> concurrent waiters which for now does not cause a latency problem. Keep it
> simple for now.
>
> The fixup of the warning check in the USB gadget driver is just a straight
> forward conversion of the lockless waiter check from one waitqueue type to
> the other.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>

Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>