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    SubjectRe: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer
    On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
    > @@ -650,6 +686,33 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
    > struct rwsem_waiter waiter;
    > DEFINE_WAKE_Q(wake_q);
    >
    > + if (!rwsem_can_spin_on_owner(sem))
    > + goto queue;
    > +
    > + /*
    > + * Undo read bias from down_read() and do optimistic spinning.
    > + */
    > + atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
    > + adjustment = 0;
    > + if (rwsem_optimistic_spin(sem, false)) {
    > + unsigned long flags;
    > +
    > + /*
    > + * Opportunistically wake up other readers in the wait queue.
    > + * It has another chance of wakeup at unlock time.
    > + */
    > + if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS) &&
    > + raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {

    why trylock? Also the rest of this function uses _irq(). Having had to
    define @flags should've been a clue.

    You simply cnanot do down_*() with IRQs disabled.

    > + if (!list_empty(&sem->wait_list))
    > + __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
    > + &wake_q);
    > + raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    > + wake_up_q(&wake_q);
    > + }
    > + return sem;
    > + }
    > +
    > +queue:
    > waiter.task = current;
    > waiter.type = RWSEM_WAITING_FOR_READ;
    > waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;

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