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

[Julian Calaby <julian.calaby@gmail.com>]

Hi Thomas,

On Thu, Mar 19, 2020 at 7:48 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> From: Thomas Gleixner <tglx@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@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> ---
> V2: Split out the orinoco and usb gadget parts and amended change log
> ---
>  drivers/usb/gadget/function/f_fs.c |    2 +-
>  include/linux/completion.h         |    8 ++++----
>  kernel/sched/completion.c          |   36 +++++++++++++++++++-----------------
>  3 files changed, 24 insertions(+), 22 deletions(-)
>
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -1703,7 +1703,7 @@ static void ffs_data_put(struct ffs_data
>                 pr_info("%s(): freeing\n", __func__);
>                 ffs_data_clear(ffs);
>                 BUG_ON(waitqueue_active(&ffs->ev.waitq) ||
> -                      waitqueue_active(&ffs->ep0req_completion.wait) ||
> +                      swait_active(&ffs->ep0req_completion.wait) ||

This looks like some code is reaching deep into the dirty dark corners
of the completion implementation, should there be some wrapper around
this to hide that?

Thanks,

-- 
Julian Calaby

Email: julian.calaby@gmail.com
Profile: http://www.google.com/profiles/julian.calaby/