[PARISC] Improve rwlock implementation

Rewrite rwlock implementation to avoid various deadlocks in the current
scheme.
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

include/asm-parisc/spinlock.h

@@ -56,50 +56,70 @@ static inline int __raw_spin_trylock(raw_spinlock_t *x)
 }
 
 /*
- * Read-write spinlocks, allowing multiple readers
- * but only one writer.
+ * Read-write spinlocks, allowing multiple readers but only one writer.
+ * The spinlock is held by the writer, preventing any readers or other
+ * writers from grabbing the rwlock.  Readers use the lock to serialise their
+ * access to the counter (which records how many readers currently hold the
+ * lock).  Linux rwlocks are unfair to writers; they can be starved for
+ * an indefinite time by readers.  They can also be taken in interrupt context,
+ * so we have to disable interrupts when acquiring the spin lock to be sure
+ * that an interrupting reader doesn't get an inconsistent view of the lock.
  */
 
-#define __raw_read_trylock(lock) generic__raw_read_trylock(lock)
-
-/* read_lock, read_unlock are pretty straightforward.  Of course it somehow
- * sucks we end up saving/restoring flags twice for read_lock_irqsave aso. */
-
 static  __inline__ void __raw_read_lock(raw_rwlock_t *rw)
 {
+	unsigned long flags;
+	local_irq_save(flags);
 	__raw_spin_lock(&rw->lock);
-
 	rw->counter++;
-
 	__raw_spin_unlock(&rw->lock);
+	local_irq_restore(flags);
 }
 
 static  __inline__ void __raw_read_unlock(raw_rwlock_t *rw)
 {
+	unsigned long flags;
+	local_irq_save(flags);
 	__raw_spin_lock(&rw->lock);
-
 	rw->counter--;
-
 	__raw_spin_unlock(&rw->lock);
+	local_irq_restore(flags);
 }
 
-/* write_lock is less trivial.  We optimistically grab the lock and check
- * if we surprised any readers.  If so we release the lock and wait till
- * they're all gone before trying again
- *
- * Also note that we don't use the _irqsave / _irqrestore suffixes here.
- * If we're called with interrupts enabled and we've got readers (or other
- * writers) in interrupt handlers someone fucked up and we'd dead-lock
- * sooner or later anyway.   prumpf */
+static __inline__ int __raw_read_trylock(raw_rwlock_t *rw)
+{
+	unsigned long flags;
+ retry:
+	local_irq_save(flags);
+	if (__raw_spin_trylock(&rw->lock)) {
+		rw->counter++;
+		__raw_spin_unlock(&rw->lock);
+		local_irq_restore(flags);
+		return 1;
+	}
+
+	local_irq_restore(flags);
+	/* If write-locked, we fail to acquire the lock */
+	if (rw->counter < 0)
+		return 0;
+
+	/* Wait until we have a realistic chance at the lock */
+	while (__raw_spin_is_locked(&rw->lock) && rw->counter >= 0)
+		cpu_relax();
+
+	goto retry;
+}
 
-static  __inline__ void __raw_write_lock(raw_rwlock_t *rw)
+static __inline__ void __raw_write_lock(raw_rwlock_t *rw)
 {
+	unsigned long flags;
 retry:
+	local_irq_save(flags);
 	__raw_spin_lock(&rw->lock);
 
-	if(rw->counter != 0) {
-		/* this basically never happens */
+	if (rw->counter != 0) {
 		__raw_spin_unlock(&rw->lock);
+		local_irq_restore(flags);
 
 		while (rw->counter != 0)
 			cpu_relax();
@@ -107,31 +127,35 @@ static  __inline__ void __raw_write_lock(raw_rwlock_t *rw)
 		goto retry;
 	}
 
-	/* got it.  now leave without unlocking */
-	rw->counter = -1; /* remember we are locked */
+	rw->counter = -1; /* mark as write-locked */
+	mb();
+	local_irq_restore(flags);
 }
 
-/* write_unlock is absolutely trivial - we don't have to wait for anything */
-
-static  __inline__ void __raw_write_unlock(raw_rwlock_t *rw)
+static __inline__ void __raw_write_unlock(raw_rwlock_t *rw)
 {
 	rw->counter = 0;
 	__raw_spin_unlock(&rw->lock);
 }
 
-static  __inline__ int __raw_write_trylock(raw_rwlock_t *rw)
+static __inline__ int __raw_write_trylock(raw_rwlock_t *rw)
 {
-	__raw_spin_lock(&rw->lock);
-	if (rw->counter != 0) {
-		/* this basically never happens */
-		__raw_spin_unlock(&rw->lock);
-
-		return 0;
+	unsigned long flags;
+	int result = 0;
+
+	local_irq_save(flags);
+	if (__raw_spin_trylock(&rw->lock)) {
+		if (rw->counter == 0) {
+			rw->counter = -1;
+			result = 1;
+		} else {
+			/* Read-locked.  Oh well. */
+			__raw_spin_unlock(&rw->lock);
+		}
 	}
+	local_irq_restore(flags);
 
-	/* got it.  now leave without unlocking */
-	rw->counter = -1; /* remember we are locked */
-	return 1;
+	return result;
 }
 
 /*