Messages in this thread |  | | Date | Wed, 5 Oct 2016 08:19:22 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH-tip v4 01/10] locking/osq: Make lock/unlock proper acquire/release barrier |
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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
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