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    SubjectRe: [patch V2 11/15] completion: Use simple wait queues
    On Wed, Mar 18, 2020 at 09:43:13PM +0100, Thomas Gleixner 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(-)

    For USB portion:

    Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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