Re: [RFC PATCH-tip v4 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

[Waiman Long]

On 10/04/2016 03:06 PM, Davidlohr Bueso wrote:
> On Thu, 18 Aug 2016, Waiman Long wrote:
>
> > The osq_lock() and osq_unlock() function may not provide the necessary
> > acquire and release barrier in some cases. This patch makes sure
> > that the proper barriers are provided when osq_lock() is successful
> > or when osq_unlock() is called.
>
> But why do we need these guarantees given that osq is only used 
> internally
> for lock owner spinning situations? Leaking out of the critical region 
> will
> obviously be bad if using it as a full lock, but, as is, this can only 
> hurt
> performance of two of the most popular locks in the kernel -- although 
> yes,
> using smp_acquire__after_ctrl_dep is nicer for polling.

First of all, it is not obvious from the name osq_lock() that it is not 
an acquire barrier in some cases. We either need to clearly document it 
or has a variant name that indicate that, e.g. osq_lock_relaxed, for 
example.

Secondly, if we look at the use cases of osq_lock(), the additional 
latency (for non-x86 archs) only matters if the master lock is 
immediately available for acquisition after osq_lock() return. 
Otherwise, it will be hidden in the spinning loop for that master lock. 
So yes, there may be a slight performance hit in some cases, but 
certainly not always.

> If you need tighter osq for rwsems, could it be refactored such that 
> mutexes
> do not take a hit?

Yes, we can certainly do that like splitting into 2 variants, one with 
acquire barrier guarantee and one without.

> > Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
> > ---
> > kernel/locking/osq_lock.c |   24 ++++++++++++++++++------
> > 1 files changed, 18 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index 05a3785..3da0b97 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -124,6 +124,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >
> >         cpu_relax_lowlatency();
> >     }
> > +    /*
> > +     * Add an acquire memory barrier for pairing with the release 
> > barrier
> > +     * in unlock.
> > +     */
> > +    smp_acquire__after_ctrl_dep();
> >     return true;
> >
> > unqueue:
> > @@ -198,13 +203,20 @@ void osq_unlock(struct optimistic_spin_queue 
> > *lock)
> >      * Second most likely case.
> >      */
> >     node = this_cpu_ptr(&osq_node);
> > -    next = xchg(&node->next, NULL);
> > -    if (next) {
> > -        WRITE_ONCE(next->locked, 1);
> > +    next = xchg_relaxed(&node->next, NULL);
> > +    if (next)
> > +        goto unlock;
> > +
> > +    next = osq_wait_next(lock, node, NULL);
> > +    if (unlikely(!next)) {
> > +        /*
> > +         * In the unlikely event that the OSQ is empty, we need to
> > +         * provide a proper release barrier.
> > +         */
> > +        smp_mb();
> >         return;
> >     }
> >
> > -    next = osq_wait_next(lock, node, NULL);
> > -    if (next)
> > -        WRITE_ONCE(next->locked, 1);
> > +unlock:
> > +    smp_store_release(&next->locked, 1);
> > }
>
> As well as for the smp_acquire__after_ctrl_dep comment you have above, 
> this also
> obviously pairs with the osq_lock's smp_load_acquire while backing out 
> (unqueueing,
> step A). Given the above, for this case we might also just rely on 
> READ_ONCE(node->locked),
> if we get the conditional wrong and miss the node becoming locked, all 
> we do is another
> iteration, and while there is a cmpxchg() there, it is mitigated with 
> the ccas thingy.

Similar to osq_lock(), the current osq_unlock() does not have a release 
barrier guarantee. I think splitting into 2 variants - osq_unlock, 
osq_unlock_relaxed will help.

Cheers,
Longman