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

Waiman Long
2016-10-05T12:19:22Z

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@infradead.org>
>> Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
>> ---
>> 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