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

[Boqun Feng]

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>
---
Hi Thomas and Peter,

Sorry I'm late for the party of PREEMPT_RT lock review. Just want to
point the problem with this patch. Not even compile test, but show the
idea and check if I'm missing something subtle.

Regards,
Boqun


 kernel/locking/rwbase_rt.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

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.
+ *
  * 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;) {
+		/* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
 		if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
 			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);
 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 	rwbase_rtmutex_unlock(rtm);
 }
@@ -216,8 +229,14 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
 	 */
 	rwbase_set_and_save_current_state(state);
 
-	/* Block until all readers have left the critical section. */
-	for (; atomic_read(&rwb->readers);) {
+	/*
+	 * Block until all readers have left the critical section.
+	 *
+	 * _acqurie() is needed in case that the reader side runs in the fast
+	 * path, pairing with the atomic_dec_and_test() in rwbase_read_unlock(),
+	 * provides ACQUIRE.
+	 */
+	for (; atomic_read_acquire(&rwb->readers);) {
 		/* Optimized out for rwlocks */
 		if (rwbase_signal_pending_state(state, current)) {
 			__set_current_state(TASK_RUNNING);
@@ -229,6 +248,9 @@ 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.
+		 *
+		 * _acquire() is not needed, because we can rely on the smp_mb()
+		 * in set_current_state() to provide ACQUIRE.
 		 */
 		if (atomic_read(&rwb->readers) != 0)
 			rwbase_schedule();
@@ -253,7 +275,11 @@ 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)) {
+	/*
+	 * _acquire() is needed in case reader is in the fast path, pairing with
+	 * rwbase_read_unlock(), provides ACQUIRE.
+	 */
+	if (!atomic_read_acquire(&rwb->readers)) {
 		atomic_set(&rwb->readers, WRITER_BIAS);
 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 		return 1;
-- 
2.32.0