From 6e071852a10ec02570c472052f07b5facb5ad857 Mon Sep 17 00:00:00 2001 From: Matthew Wilcox Date: Sat, 2 Sep 2006 07:54:58 -0600 Subject: [PARISC] Improve rwlock implementation Rewrite rwlock implementation to avoid various deadlocks in the current scheme. Signed-off-by: Matthew Wilcox Signed-off-by: Kyle McMartin --- include/asm-parisc/spinlock.h | 98 +++++++++++++++++++++++++++---------------- 1 file changed, 61 insertions(+), 37 deletions(-) (limited to 'include') diff --git a/include/asm-parisc/spinlock.h b/include/asm-parisc/spinlock.h index e1825530365d..8980a876cc4e 100644 --- a/include/asm-parisc/spinlock.h +++ b/include/asm-parisc/spinlock.h @@ -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) +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 @@ 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) +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); - - return 0; + 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); + } } + local_irq_restore(flags); - /* got it. now leave without unlocking */ - rw->counter = -1; /* remember we are locked */ - return 1; + return result; } /* -- cgit v1.2.3-55-g7522