From fbdc8f0f4891df7b5eb643ec0a509a4ac7dcfc2e Mon Sep 17 00:00:00 2001 From: Helge Deller Date: Sun, 5 Apr 2020 22:29:55 +0200 Subject: parisc: Rework arch_rw locking functions Clean up the arch read/write locking functions based on the arc implemenation. This improves readability of those functions. Signed-off-by: Helge Deller --- arch/parisc/include/asm/spinlock.h | 135 +++++++++++++------------------ arch/parisc/include/asm/spinlock_types.h | 14 +++- 2 files changed, 67 insertions(+), 82 deletions(-) (limited to 'arch') 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 -- cgit