Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write

From: Jason Low
Date: Tue Apr 28 2015 - 13:50:10 EST


On Tue, 2015-04-28 at 19:17 +0200, Peter Zijlstra wrote:

> To me it makes more sense to reverse these two branches (identical code
> wise of course) and put the special case first.
>
> Alternatively we could also do something like the below, which to my
> eyes looks a little better still, but I don't care too much.
>
> if (rwsem_has_spinner(sem)) {
> /*
> * comment ...
> */
> smp_rmb();
> if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> return sem;
> goto locked;
> }
>
> raw_spin_lock_irqsave(&sem->wait_lock, flags);
> locked:

How about putting this into its own function:

static inline bool __rwsem_wake_acquire_wait_lock(sem)
{
/*
*
* Comments
*
*/
if (unlikely(rwsem_has_spinner(sem))) {
/*
* Comments
*/
smp_rmb();
if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
return false;
}

return true;
}


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