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

[Davidlohr Bueso]

On Wed, 18 Mar 2020, Thomas Gleixner wrote:

>From: Thomas Gleixner <tglx at linutronix.de>
>
>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 at linutronix.de>
>Cc: Arnd Bergmann <arnd at arndb.de>

Reviewed-by: Davidlohr Bueso <dbueso at suse.de>