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. The rcu_read_lock prevents
owner from getting freed. The comment's intention is to warn that
owner->on_cpu may not be reliable.
I'm using similar logic in rw-sem.
Tim
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