Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lockacquisition

[Peter Zijlstra]

On Mon, Jun 24, 2013 at 01:17:45PM -0700, Tim Chen wrote:
> On second thought, I agree with you.  I should change this to
> something like
> 
> 	int retval = true;
> 	task_struct *sem_owner;
> 
> 	/* Spin only if active writer running */
> 	if (!sem->owner)
> 		return false;
> 
> 	rcu_read_lock();
> 	sem_owner = sem->owner;

That should be: sem_owner = ACCESS_ONCE(sem->owner); to make sure the
compiler doesn't try and be clever and rereads.

> 	if (sem_owner)
> 		retval = sem_owner->on_cpu;
> 
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