parisc: Rework arch_rw locking functions

Clean up the arch read/write locking functions based on the arc
implemenation. This improves readability of those functions.

Signed-off-by: Helge Deller <[e-mail]>

arch/parisc/include/asm/spinlock.h

···
 
 /*
  * Read-write spinlocks, allowing multiple readers but only one writer.
- * Linux rwlocks are unfair to writers; they can be starved for an indefinite
- * time by readers.  With care, they can also be taken in interrupt context.
+ * Unfair locking as Writers could be starved indefinitely by Reader(s)
  *
- * In the PA-RISC implementation, we have a spinlock and a counter.
- * Readers use the lock to serialise their access to the counter (which
- * records how many readers currently hold the lock).
- * Writers hold the spinlock, preventing any readers or other writers from
- * grabbing the rwlock.
+ * The spinlock itself is contained in @counter and access to it is
+ * serialized with @lock_mutex.
  */
 
-/* Note that we have to ensure interrupts are disabled in case we're
- * interrupted by some other code that wants to grab the same read lock */
-static  __inline__ void arch_read_lock(arch_rwlock_t *rw)
+/* 1 - lock taken successfully */
+static inline int arch_read_trylock(arch_rwlock_t *rw)
 {
+	int ret = 0;
 	unsigned long flags;
-	local_irq_save(flags);
-	arch_spin_lock_flags(&rw->lock, flags);
-	rw->counter++;
-	arch_spin_unlock(&rw->lock);
-	local_irq_restore(flags);
-}
 
-/* Note that we have to ensure interrupts are disabled in case we're
- * interrupted by some other code that wants to grab the same read lock */
-static  __inline__ void arch_read_unlock(arch_rwlock_t *rw)
-{
-	unsigned long flags;
 	local_irq_save(flags);
-	arch_spin_lock_flags(&rw->lock, flags);
-	rw->counter--;
-	arch_spin_unlock(&rw->lock);
-	local_irq_restore(flags);
-}
+	arch_spin_lock(&(rw->lock_mutex));
 
-/* Note that we have to ensure interrupts are disabled in case we're
- * interrupted by some other code that wants to grab the same read lock */
-static __inline__ int arch_read_trylock(arch_rwlock_t *rw)
-{
-	unsigned long flags;
- retry:
-	local_irq_save(flags);
-	if (arch_spin_trylock(&rw->lock)) {
-		rw->counter++;
-		arch_spin_unlock(&rw->lock);
-		local_irq_restore(flags);
-		return 1;
+	/*
+	 * zero means writer holds the lock exclusively, deny Reader.
+	 * Otherwise grant lock to first/subseq reader
+	 */
+	if (rw->counter > 0) {
+		rw->counter--;
+		ret = 1;
 	}
 
+	arch_spin_unlock(&(rw->lock_mutex));
 	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 (arch_spin_is_locked(&rw->lock) && rw->counter >= 0)
+	return ret;
+}
+
+/* 1 - lock taken successfully */
+static inline int arch_write_trylock(arch_rwlock_t *rw)
+{
+	int ret = 0;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	arch_spin_lock(&(rw->lock_mutex));
+
+	/*
+	 * If reader(s) hold lock (lock < __ARCH_RW_LOCK_UNLOCKED__),
+	 * deny writer. Otherwise if unlocked grant to writer
+	 * Hence the claim that Linux rwlocks are unfair to writers.
+	 * (can be starved for an indefinite time by readers).
+	 */
+	if (rw->counter == __ARCH_RW_LOCK_UNLOCKED__) {
+		rw->counter = 0;
+		ret = 1;
+	}
+	arch_spin_unlock(&(rw->lock_mutex));
+	local_irq_restore(flags);
+
+	return ret;
+}
+
+static inline void arch_read_lock(arch_rwlock_t *rw)
+{
+	while (!arch_read_trylock(rw))
 		cpu_relax();
-
-	goto retry;
 }
 
-/* Note that we have to ensure interrupts are disabled in case we're
- * interrupted by some other code that wants to read_trylock() this lock */
-static __inline__ void arch_write_lock(arch_rwlock_t *rw)
+static inline void arch_write_lock(arch_rwlock_t *rw)
+{
+	while (!arch_write_trylock(rw))
+		cpu_relax();
+}
+
+static inline void arch_read_unlock(arch_rwlock_t *rw)
 {
 	unsigned long flags;
-retry:
+
 	local_irq_save(flags);
-	arch_spin_lock_flags(&rw->lock, flags);
-
-	if (rw->counter != 0) {
-		arch_spin_unlock(&rw->lock);
-		local_irq_restore(flags);
-
-		while (rw->counter != 0)
-			cpu_relax();
-
-		goto retry;
-	}
-
-	rw->counter = -1; /* mark as write-locked */
-	mb();
+	arch_spin_lock(&(rw->lock_mutex));
+	rw->counter++;
+	arch_spin_unlock(&(rw->lock_mutex));
 	local_irq_restore(flags);
 }
 
-static __inline__ void arch_write_unlock(arch_rwlock_t *rw)
-{
-	rw->counter = 0;
-	arch_spin_unlock(&rw->lock);
-}
-
-/* Note that we have to ensure interrupts are disabled in case we're
- * interrupted by some other code that wants to read_trylock() this lock */
-static __inline__ int arch_write_trylock(arch_rwlock_t *rw)
+static inline void arch_write_unlock(arch_rwlock_t *rw)
 {
 	unsigned long flags;
-	int result = 0;
 
 	local_irq_save(flags);
-	if (arch_spin_trylock(&rw->lock)) {
-		if (rw->counter == 0) {
-			rw->counter = -1;
-			result = 1;
-		} else {
-			/* Read-locked.  Oh well. */
-			arch_spin_unlock(&rw->lock);
-		}
-	}
+	arch_spin_lock(&(rw->lock_mutex));
+	rw->counter = __ARCH_RW_LOCK_UNLOCKED__;
+	arch_spin_unlock(&(rw->lock_mutex));
 	local_irq_restore(flags);
-
-	return result;
 }
 
 #endif /* __ASM_SPINLOCK_H */

arch/parisc/include/asm/spinlock_types.h

···
 #endif
 } arch_spinlock_t;
 
+
+/* counter:
+ * Unlocked     : 0x0100_0000
+ * Read lock(s) : 0x00FF_FFFF to 0x01  (Multiple Readers decrement it)
+ * Write lock   : 0x0, but only if prior value is "unlocked" 0x0100_0000
+ */
 typedef struct {
-	arch_spinlock_t lock;
-	volatile int counter;
+	arch_spinlock_t		lock_mutex;
+	volatile unsigned int	counter;
 } arch_rwlock_t;
 
-#define __ARCH_RW_LOCK_UNLOCKED		{ __ARCH_SPIN_LOCK_UNLOCKED, 0 }
+#define __ARCH_RW_LOCK_UNLOCKED__       0x01000000
+#define __ARCH_RW_LOCK_UNLOCKED         { .lock_mutex = __ARCH_SPIN_LOCK_UNLOCKED, \
+					.counter = __ARCH_RW_LOCK_UNLOCKED__ }
 
 #endif