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SubjectRe: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader
On Wed, 01 Sep 2021, Boqun Feng wrote:
>diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
>index 4ba15088e640..a1886fd8bde6 100644
>--- a/kernel/locking/rwbase_rt.c
>+++ b/kernel/locking/rwbase_rt.c
>@@ -41,6 +41,12 @@
> * The risk of writer starvation is there, but the pathological use cases
> * which trigger it are not necessarily the typical RT workloads.
> *
>+ * Fast-path orderings:
>+ * The lock/unlock of readers can run in fast paths: lock and unlock are only
>+ * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
>+ * semantics of rwbase_rt. Atomic ops then should be stronger than _acquire()
>+ * and _release() to provide necessary ordering guarantee.

Perhaps the following instead?

+ * Ordering guarantees: As with any locking primitive, (load)-ACQUIRE and
+ * (store)-RELEASE semantics are guaranteed for lock and unlock operations,
+ * respectively; such that nothing leaks out of the critical region. When
+ * writers are involved this is provided through the rtmutex. However, for
+ * reader fast-paths, the atomics provide at least such guarantees.

Also, I think you could remove most of the comments wrt _acquire or _release
in the fastpath for each ->readers atomic op, unless it isn't obvious.

>+ *
> * Common code shared between RT rw_semaphore and rwlock
> */
>
>@@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
> * set.
> */
> for (r = atomic_read(&rwb->readers); r < 0;) {

Unrelated, but we probably wanna get rid of these abusing for-loops throughout.

>+ /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
> if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))

As Waiman suggested, this can be _acquire() - albeit we're only missing
an L->L for acquire semantics upon returning, per the control dependency
already guaranteeing L->S. That way we would loop with _relaxed().

> return 1;
> }
>@@ -162,6 +169,8 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
> /*
> * rwb->readers can only hit 0 when a writer is waiting for the
> * active readers to leave the critical section.
>+ *
>+ * dec_and_test() is fully ordered, provides RELEASE.
> */
> if (unlikely(atomic_dec_and_test(&rwb->readers)))
> __rwbase_read_unlock(rwb, state);
>@@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
> {
> struct rt_mutex_base *rtm = &rwb->rtmutex;
>
>- atomic_add(READER_BIAS - bias, &rwb->readers);
>+ /*
>+ * _release() is needed in case that reader is in fast path, pairing
>+ * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
>+ */
>+ (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);

Hmmm while defined, there are no users atomic_add_return_release (yet?). I think
this is because the following is preferred when the return value is not really
wanted, but only the Rmw ordering it provides:

+ smp_mb__before_atomic(); /* provide RELEASE semantics */
atomic_add(READER_BIAS - bias, &rwb->readers);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
rwbase_rtmutex_unlock(rtm);

> raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> rwbase_rtmutex_unlock(rtm);
> }

Thanks,
Davidlohr

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