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

[Peter Hurley]

On 06/24/2013 03:13 PM, Tim Chen wrote:
> On Mon, 2013-06-24 at 14:49 -0400, Peter Hurley wrote:
> > 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.
> >      */
>
> In mutex_spin_on_owner, after rcu_read_lock, the owner_running()
> function de-references the owner pointer.

Only after establishing the following preconditions:
1. snapshot of owner is non-NULL
2. mutex->owner == snapshot owner
3. memory holding mutex has not been freed (that's what the
   rcu_read_lock() is for)

Only then is the owner dereferenced and only through the snapshot
(not the now-possibly-rewritten sem->owner).

> I'm using similar logic in rw-sem.

With crucial details absent.

Regards,
Peter Hurley


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