Clean up the arch read/write locking functions based on the arc
implemenation. This improves readability of those functions.
Signed-off-by: Helge Deller <deller@xxxxxx>
diff --git a/arch/parisc/include/asm/spinlock.h b/arch/parisc/include/asm/spinlock.h
index 51582eacb7ec..70fecb8dc4e2 100644
--- a/arch/parisc/include/asm/spinlock.h
+++ b/arch/parisc/include/asm/spinlock.h
@@ -67,116 +67,93 @@ static inline int arch_spin_trylock(arch_spinlock_t *x)
/*
* 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);
+ arch_spin_lock(&(rw->lock_mutex));
+
+ /*
+ * 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);
+
+ return ret;
}
-/* 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)
+/* 1 - lock taken successfully */
+static inline int arch_write_trylock(arch_rwlock_t *rw)
{
+ int ret = 0;
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;
+ 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);
- /* 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;
+}
+
+static inline void arch_read_lock(arch_rwlock_t *rw)
+{
+ while (!arch_read_trylock(rw))
cpu_relax();
+}
- goto retry;
+static inline void arch_write_lock(arch_rwlock_t *rw)
+{
+ while (!arch_write_trylock(rw))
+ cpu_relax();
}
-/* 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_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();
+ local_irq_save(flags);
+ 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 */
diff --git a/arch/parisc/include/asm/spinlock_types.h b/arch/parisc/include/asm/spinlock_types.h
index 42979c5704dc..ca39ee350c3f 100644
--- a/arch/parisc/include/asm/spinlock_types.h
+++ b/arch/parisc/include/asm/spinlock_types.h
@@ -12,11 +12,19 @@ typedef struct {
#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
