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

[Peter Hurley]

On 06/24/2013 01:11 PM, Tim Chen wrote:
> On Sun, 2013-06-23 at 13:03 -0700, Davidlohr Bueso wrote:
> > On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
> > > On 06/21/2013 07:51 PM, Tim Chen wrote:
> > > > +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > > > +{
> > > > +	int retval = true;
> > > > +
> > > > +	/* Spin only if active writer running */
> > > > +	if (!sem->owner)
> > > > +		return false;
> > > > +
> > > > +	rcu_read_lock();
> > > > +	if (sem->owner)
> > > > +		retval = sem->owner->on_cpu;
> > >                            ^^^^^^^^^^^^^^^^^^
> > >
> > > Why is this a safe dereference? Could not another cpu have just
> > > dropped the sem (and thus set sem->owner to NULL and oops)?
>
> The rcu read lock should protect against sem->owner being NULL.

It doesn't.

Here's the comment from mutex_spin_on_owner():

  /*
   * Look out! "owner" is an entirely speculative pointer
   * access and not reliable.
   */

Regards,
Peter Hurley

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