Commit 6e071852 authored by Matthew Wilcox's avatar Matthew Wilcox
Browse files

[PARISC] Improve rwlock implementation 



Rewrite rwlock implementation to avoid various deadlocks in the current
scheme.

Signed-off-by: default avatarMatthew Wilcox <matthew@wil.cx>
Signed-off-by: default avatarKyle McMartin <kyle@parisc-linux.org>
parent 9c2c5457

include/asm-parisc/spinlock.h

+61 −37
Original line number Diff line number Diff line
@@ -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);

}



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;

	}



/* 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 */

	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 @@ 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)

{

	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;

}



/*