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[PARISC] Improve rwlock implementation

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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>
This commit is contained in:
Matthew Wilcox 2006-09-02 07:54:58 -06:00 committed by Matthew Wilcox
parent 9c2c54574e
commit 6e071852a1
1 changed files with 61 additions and 37 deletions
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88
include/asm-parisc/spinlock.h
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@ -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)
{
unsigned long flags;
retry:
local_irq_save(flags);
__raw_spin_lock(&rw->lock);
if (rw->counter != 0) {
/* this basically never happens */
__raw_spin_unlock(&rw->lock);
local_irq_restore(flags);
while (rw->counter != 0)
cpu_relax();
@ -107,12 +127,11 @@ retry:
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)
{
rw->counter = 0;
@ -121,17 +140,22 @@ static __inline__ void __raw_write_unlock(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 */
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);
return 0;
}
}
local_irq_restore(flags);
/* got it. now leave without unlocking */
rw->counter = -1; /* remember we are locked */
return 1;
return result;
}
/*