[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>
diff --git a/include/asm-parisc/spinlock.h b/include/asm-parisc/spinlock.h
index e182553..8980a87 100644
--- a/include/asm-parisc/spinlock.h
+++ b/include/asm-parisc/spinlock.h
@@ -56,50 +56,70 @@
}
/*
- * 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__ void __raw_write_lock(raw_rwlock_t *rw)
+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)
+{
+ 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 @@
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);
+ unsigned long flags;
+ int result = 0;
- return 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;
}
/*