[Patch v3] rwsem: fix rwsem_is_locked() bugs

[Amerigo Wang]

rwsem_is_locked() tests ->activity without locks, so we should always
keep ->activity consistent. However, the code in __rwsem_do_wake()
breaks this rule, it updates ->activity after _all_ readers waken up,
this may give some reader a wrong ->activity value, thus cause
rwsem_is_locked() behaves wrong.

Quote from Andrew:

"
- we have one or more processes sleeping in down_read(), waiting for access.

- we wake one or more processes up without altering ->activity

- they start to run and they do rwsem_is_locked().  This incorrectly
  returns "false", because the waker process is still crunching away in
  __rwsem_do_wake().

- the waker now alters ->activity, but it was too late.

And the patch fixes this by updating ->activity prior to waking the
sleeping processes.  So when they run, they'll see a non-zero value of
->activity.
"

Also, we have more problems, as pointed by David:

"... the case where the active readers run out, but there's a
writer on the queue (see __up_read()), nor the case where the active writer
ends, but there's a waiter on the queue (see __up_write()).  In both cases,
the lock is still held, though sem->activity is 0."

This patch fixes this too.

David also said we may have "the potential to cause more cacheline ping-pong
under contention", but "this change shouldn't cause a significant slowdown."

With this patch applied, I can't trigger that bug any more.

Reported-by: Brian Behlendorf <behlendorf1@xxxxxxxx>
Cc: Ben Woodard <bwoodard@xxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>

---
diff --git a/include/linux/rwsem-spinlock.h b/include/linux/rwsem-spinlock.h
index 6c3c0f6..1a65776 100644
--- a/include/linux/rwsem-spinlock.h
+++ b/include/linux/rwsem-spinlock.h
@@ -71,7 +71,14 @@ extern void __downgrade_write(struct rw_semaphore *sem);
 
 static inline int rwsem_is_locked(struct rw_semaphore *sem)
 {
-	return (sem->activity != 0);
+	int ret;
+
+	if (spin_trylock_irq(&sem->wait_lock)) {
+		ret = !(list_empty(&sem->wait_list) && sem->activity == 0);
+		spin_unlock_irq(&sem->wait_lock);
+		return ret;
+	}
+	return 1;
 }
 
 #endif /* __KERNEL__ */
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index 9df3ca5..234d83f 100644
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -78,7 +78,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 	/* grant an infinite number of read locks to the front of the queue */
  dont_wake_writers:
-	woken = 0;
+	/*
+	 * we increase ->activity just to make rwsem_is_locked() happy,
+	 * to avoid potential cache line ping-pong, we don't do this
+	 * within the following loop.
+	 */
+	woken = sem->activity++;
 	while (waiter->flags & RWSEM_WAITING_FOR_READ) {
 		struct list_head *next = waiter->list.next;
 
@@ -94,7 +99,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 		waiter = list_entry(next, struct rwsem_waiter, list);
 	}
 
-	sem->activity += woken;
+	sem->activity = woken;
 
  out:
 	return sem;
--
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