![Martin Schwidefsky](/assets/img/avatar_default.png)
On systems running with virtual cpus there is optimization potential in regard to spinlocks and rw-locks. If the virtual cpu that has taken a lock is known to a cpu that wants to acquire the same lock it is beneficial to yield the timeslice of the virtual cpu in favour of the cpu that has the lock (directed yield). With CONFIG_PREEMPT="n" this can be implemented by the architecture without common code changes. Powerpc already does this. With CONFIG_PREEMPT="y" the lock loops are coded with _raw_spin_trylock, _raw_read_trylock and _raw_write_trylock in kernel/spinlock.c. If the lock could not be taken cpu_relax is called. A directed yield is not possible because cpu_relax doesn't know anything about the lock. To be able to yield the lock in favour of the current lock holder variants of cpu_relax for spinlocks and rw-locks are needed. The new _raw_spin_relax, _raw_read_relax and _raw_write_relax primitives differ from cpu_relax insofar that they have an argument: a pointer to the lock structure. Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Paul Mackerras <paulus@samba.org> Cc: Haavard Skinnemoen <hskinnemoen@atmel.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
213 lines
4.9 KiB
C
213 lines
4.9 KiB
C
#ifndef __ASM_SPINLOCK_H
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#define __ASM_SPINLOCK_H
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#include <asm/atomic.h>
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#include <asm/rwlock.h>
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#include <asm/page.h>
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#include <asm/processor.h>
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#include <linux/compiler.h>
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#define CLI_STRING "cli"
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#define STI_STRING "sti"
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/*
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* Your basic SMP spinlocks, allowing only a single CPU anywhere
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*
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* Simple spin lock operations. There are two variants, one clears IRQ's
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* on the local processor, one does not.
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*
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* We make no fairness assumptions. They have a cost.
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*
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* (the type definitions are in asm/spinlock_types.h)
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*/
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static inline int __raw_spin_is_locked(raw_spinlock_t *x)
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{
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return *(volatile signed char *)(&(x)->slock) <= 0;
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}
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static inline void __raw_spin_lock(raw_spinlock_t *lock)
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{
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asm volatile("\n1:\t"
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LOCK_PREFIX " ; decb %0\n\t"
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"jns 3f\n"
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"2:\t"
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"rep;nop\n\t"
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"cmpb $0,%0\n\t"
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"jle 2b\n\t"
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"jmp 1b\n"
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"3:\n\t"
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: "+m" (lock->slock) : : "memory");
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}
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/*
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* It is easier for the lock validator if interrupts are not re-enabled
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* in the middle of a lock-acquire. This is a performance feature anyway
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* so we turn it off:
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*
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* NOTE: there's an irqs-on section here, which normally would have to be
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* irq-traced, but on CONFIG_TRACE_IRQFLAGS we never use this variant.
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*/
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#ifndef CONFIG_PROVE_LOCKING
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static inline void __raw_spin_lock_flags(raw_spinlock_t *lock, unsigned long flags)
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{
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asm volatile(
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"\n1:\t"
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LOCK_PREFIX " ; decb %0\n\t"
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"jns 5f\n"
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"2:\t"
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"testl $0x200, %1\n\t"
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"jz 4f\n\t"
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STI_STRING "\n"
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"3:\t"
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"rep;nop\n\t"
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"cmpb $0, %0\n\t"
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"jle 3b\n\t"
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CLI_STRING "\n\t"
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"jmp 1b\n"
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"4:\t"
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"rep;nop\n\t"
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"cmpb $0, %0\n\t"
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"jg 1b\n\t"
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"jmp 4b\n"
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"5:\n\t"
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: "+m" (lock->slock) : "r" (flags) : "memory");
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}
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#endif
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static inline int __raw_spin_trylock(raw_spinlock_t *lock)
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{
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char oldval;
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asm volatile(
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"xchgb %b0,%1"
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:"=q" (oldval), "+m" (lock->slock)
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:"0" (0) : "memory");
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return oldval > 0;
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}
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/*
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* __raw_spin_unlock based on writing $1 to the low byte.
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* This method works. Despite all the confusion.
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* (except on PPro SMP or if we are using OOSTORE, so we use xchgb there)
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* (PPro errata 66, 92)
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*/
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#if !defined(CONFIG_X86_OOSTORE) && !defined(CONFIG_X86_PPRO_FENCE)
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static inline void __raw_spin_unlock(raw_spinlock_t *lock)
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{
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asm volatile("movb $1,%0" : "+m" (lock->slock) :: "memory");
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}
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#else
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static inline void __raw_spin_unlock(raw_spinlock_t *lock)
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{
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char oldval = 1;
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asm volatile("xchgb %b0, %1"
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: "=q" (oldval), "+m" (lock->slock)
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: "0" (oldval) : "memory");
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}
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#endif
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static inline void __raw_spin_unlock_wait(raw_spinlock_t *lock)
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{
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while (__raw_spin_is_locked(lock))
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cpu_relax();
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}
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/*
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* Read-write spinlocks, allowing multiple readers
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* but only one writer.
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*
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* NOTE! it is quite common to have readers in interrupts
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* but no interrupt writers. For those circumstances we
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* can "mix" irq-safe locks - any writer needs to get a
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* irq-safe write-lock, but readers can get non-irqsafe
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* read-locks.
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*
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* On x86, we implement read-write locks as a 32-bit counter
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* with the high bit (sign) being the "contended" bit.
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*
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* The inline assembly is non-obvious. Think about it.
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*
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* Changed to use the same technique as rw semaphores. See
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* semaphore.h for details. -ben
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*
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* the helpers are in arch/i386/kernel/semaphore.c
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*/
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/**
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* read_can_lock - would read_trylock() succeed?
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* @lock: the rwlock in question.
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*/
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static inline int __raw_read_can_lock(raw_rwlock_t *x)
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{
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return (int)(x)->lock > 0;
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}
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/**
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* write_can_lock - would write_trylock() succeed?
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* @lock: the rwlock in question.
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*/
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static inline int __raw_write_can_lock(raw_rwlock_t *x)
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{
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return (x)->lock == RW_LOCK_BIAS;
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}
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static inline void __raw_read_lock(raw_rwlock_t *rw)
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{
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asm volatile(LOCK_PREFIX " subl $1,(%0)\n\t"
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"jns 1f\n"
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"call __read_lock_failed\n\t"
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"1:\n"
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::"a" (rw) : "memory");
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}
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static inline void __raw_write_lock(raw_rwlock_t *rw)
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{
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asm volatile(LOCK_PREFIX " subl $" RW_LOCK_BIAS_STR ",(%0)\n\t"
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"jz 1f\n"
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"call __write_lock_failed\n\t"
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"1:\n"
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::"a" (rw) : "memory");
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}
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static inline int __raw_read_trylock(raw_rwlock_t *lock)
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{
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atomic_t *count = (atomic_t *)lock;
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atomic_dec(count);
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if (atomic_read(count) >= 0)
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return 1;
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atomic_inc(count);
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return 0;
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}
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static inline int __raw_write_trylock(raw_rwlock_t *lock)
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{
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atomic_t *count = (atomic_t *)lock;
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if (atomic_sub_and_test(RW_LOCK_BIAS, count))
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return 1;
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atomic_add(RW_LOCK_BIAS, count);
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return 0;
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}
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static inline void __raw_read_unlock(raw_rwlock_t *rw)
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{
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asm volatile(LOCK_PREFIX "incl %0" :"+m" (rw->lock) : : "memory");
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}
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static inline void __raw_write_unlock(raw_rwlock_t *rw)
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{
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asm volatile(LOCK_PREFIX "addl $" RW_LOCK_BIAS_STR ", %0"
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: "+m" (rw->lock) : : "memory");
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}
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#define _raw_spin_relax(lock) cpu_relax()
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#define _raw_read_relax(lock) cpu_relax()
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#define _raw_write_relax(lock) cpu_relax()
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#endif /* __ASM_SPINLOCK_H */
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