Messages in this thread | | | Subject | Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer | From | Waiman Long <> | Date | Wed, 17 Apr 2019 13:51:24 -0400 |
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On 04/17/2019 10:05 AM, Peter Zijlstra wrote: > 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.
I used trylock to avoid getting stuck in the spinlock while holding a read lock on the rwsem.
You are right. I don't to use _irqsave version. Just the _irq version is enough.
Thanks, Longman
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