Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

From: Peter Zijlstra
Date: Wed Sep 08 2021 - 07:52:31 EST


On Wed, Sep 01, 2021 at 11:06:27PM +0800, Boqun Feng wrote:
> Readers of rwbase can lock and unlock without taking any inner lock, if
> that happens, we need the ordering provided by atomic operations to
> satisfy the ordering semantics of lock/unlock. Without that, considering
> the follow case:
>
> { X = 0 initially }
>
> CPU 0 CPU 1
> ===== =====
> rt_write_lock();
> X = 1
> rt_write_unlock():
> atomic_add(READER_BIAS - WRITER_BIAS, ->readers);
> // ->readers is READER_BIAS.
> rt_read_lock():
> if ((r = atomic_read(->readers)) < 0) // True
> atomic_try_cmpxchg(->readers, r, r + 1); // succeed.
> <acquire the read lock via fast path>
>
> r1 = X; // r1 may be 0, because nothing prevent the reordering
> // of "X=1" and atomic_add() on CPU 1.
>
> Therefore audit every usage of atomic operations that may happen in a
> fast path, and add necessary barriers.
>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---

Does this work for you?

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 4ba15088e640..4685d3780683 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -53,6 +53,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
* set.
*/
for (r = atomic_read(&rwb->readers); r < 0;) {
+ /* Fully ordered on success, provides ACQUIRE */
if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
return 1;
}
@@ -162,6 +163,8 @@ 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.
+ *
+ * Fully ordered, provides RELEASE.
*/
if (unlikely(atomic_dec_and_test(&rwb->readers)))
__rwbase_read_unlock(rwb, state);
@@ -172,7 +175,10 @@ 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);
+ /*
+ * Needs to provide RELEASE vs rwbase_read_trylock().
+ */
+ (void)atomic_fetch_add_release(READER_BIAS - bias, &rwb->readers);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
rwbase_rtmutex_unlock(rtm);
}
@@ -201,23 +207,30 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
unsigned long flags;
+ int readers;

/* Take the rtmutex as a first step */
if (rwbase_rtmutex_lock_state(rtm, state))
return -EINTR;

/* Force readers into slow path */
- atomic_sub(READER_BIAS, &rwb->readers);
+ readers = atomic_sub_return_relaxed(READER_BIAS, &rwb->readers);

- raw_spin_lock_irqsave(&rtm->wait_lock, flags);
/*
* set_current_state() for rw_semaphore
* current_save_and_set_rtlock_wait_state() for rwlock
*/
rwbase_set_and_save_current_state(state);
+ raw_spin_lock_irqsave(&rtm->wait_lock, flags);

- /* Block until all readers have left the critical section. */
- for (; atomic_read(&rwb->readers);) {
+ /*
+ * Block until all readers have left the critical section.
+ *
+ * In the case of !readers, the above implies TSO ordering
+ * at the very least and hence provides ACQUIRE vs the earlier
+ * atomic_sub_return_relaxed().
+ */
+ while (readers) {
/* Optimized out for rwlocks */
if (rwbase_signal_pending_state(state, current)) {
__set_current_state(TASK_RUNNING);
@@ -230,8 +243,12 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
* Schedule and wait for the readers to leave the critical
* section. The last reader leaving it wakes the waiter.
*/
- if (atomic_read(&rwb->readers) != 0)
+ readers = atomic_read(&rwb->readers);
+ if (readers != 0)
rwbase_schedule();
+ /*
+ * Implies smp_mb() and provides ACQUIRE for the !readers case.
+ */
set_current_state(state);
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
}
@@ -253,7 +270,10 @@ static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
atomic_sub(READER_BIAS, &rwb->readers);

raw_spin_lock_irqsave(&rtm->wait_lock, flags);
- if (!atomic_read(&rwb->readers)) {
+ /*
+ * Needs ACQUIRE vs rwbase_read_unlock();
+ */
+ if (!atomic_read_acquire(&rwb->readers)) {
atomic_set(&rwb->readers, WRITER_BIAS);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
return 1;