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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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