linux/include/asm-x86/spinlock.h

#ifndef _X86_SPINLOCK_H_
#define _X86_SPINLOCK_H_

#include <asm/atomic.h>
#include <asm/rwlock.h>
#include <asm/page.h>
#include <asm/processor.h>

/*
 * Your basic SMP spinlocks, allowing only a single CPU anywhere
 *
 * Simple spin lock operations.  There are two variants, one clears IRQ's
 * on the local processor, one does not.
 *
 * We make no fairness assumptions. They have a cost.
 *
 * (the type definitions are in asm/spinlock_types.h)
 */

#ifdef CONFIG_PARAVIRT
#include <asm/paravirt.h>
#else
#define CLI_STRING	"cli"
#define STI_STRING	"sti"
#define CLI_STI_CLOBBERS
#define CLI_STI_INPUT_ARGS
#endif /* CONFIG_PARAVIRT */

#ifdef CONFIG_X86_32
typedef char _slock_t;
# define LOCK_INS_DEC "decb"
# define LOCK_INS_XCH "xchgb"
# define LOCK_INS_MOV "movb"
# define LOCK_INS_CMP "cmpb"
# define LOCK_PTR_REG "a"
#else
typedef int _slock_t;
# define LOCK_INS_DEC "decl"
# define LOCK_INS_XCH "xchgl"
# define LOCK_INS_MOV "movl"
# define LOCK_INS_CMP "cmpl"
# define LOCK_PTR_REG "D"
#endif

static inline int __raw_spin_is_locked(raw_spinlock_t *lock)
{
	return *(volatile _slock_t *)(&(lock)->slock) <= 0;
}

static inline void __raw_spin_lock(raw_spinlock_t *lock)
{
	asm volatile(
		"\n1:\t"
		LOCK_PREFIX " ; " LOCK_INS_DEC " %0\n\t"
		"jns 3f\n"
		"2:\t"
		"rep;nop\n\t"
		LOCK_INS_CMP " $0,%0\n\t"
		"jle 2b\n\t"
		"jmp 1b\n"
		"3:\n\t"
		: "+m" (lock->slock) : : "memory");
}

/*
 * It is easier for the lock validator if interrupts are not re-enabled
 * in the middle of a lock-acquire. This is a performance feature anyway
 * so we turn it off:
 *
 * NOTE: there's an irqs-on section here, which normally would have to be
 * irq-traced, but on CONFIG_TRACE_IRQFLAGS we never use this variant.
 */
#ifndef CONFIG_PROVE_LOCKING
static inline void __raw_spin_lock_flags(raw_spinlock_t *lock,
					 unsigned long flags)
{
	asm volatile(
		"\n1:\t"
		LOCK_PREFIX " ; " LOCK_INS_DEC " %[slock]\n\t"
		"jns 5f\n"
		"testl $0x200, %[flags]\n\t"
		"jz 4f\n\t"
		STI_STRING "\n"
		"3:\t"
		"rep;nop\n\t"
		LOCK_INS_CMP " $0, %[slock]\n\t"
		"jle 3b\n\t"
		CLI_STRING "\n\t"
		"jmp 1b\n"
		"4:\t"
		"rep;nop\n\t"
		LOCK_INS_CMP " $0, %[slock]\n\t"
		"jg 1b\n\t"
		"jmp 4b\n"
		"5:\n\t"
		: [slock] "+m" (lock->slock)
		: [flags] "r" ((u32)flags)
		  CLI_STI_INPUT_ARGS
		: "memory" CLI_STI_CLOBBERS);
}
#endif

static inline int __raw_spin_trylock(raw_spinlock_t *lock)
{
	_slock_t oldval;

	asm volatile(
		LOCK_INS_XCH " %0,%1"
		:"=q" (oldval), "+m" (lock->slock)
		:"0" (0) : "memory");

	return oldval > 0;
}

/*
 * __raw_spin_unlock based on writing $1 to the low byte.
 * This method works. Despite all the confusion.
 * (except on PPro SMP or if we are using OOSTORE, so we use xchgb there)
 * (PPro errata 66, 92)
 */
#if defined(X86_64) || \
	(!defined(CONFIG_X86_OOSTORE) && !defined(CONFIG_X86_PPRO_FENCE))

static inline void __raw_spin_unlock(raw_spinlock_t *lock)
{
	asm volatile(LOCK_INS_MOV " $1,%0" : "=m" (lock->slock) :: "memory");
}

#else

static inline void __raw_spin_unlock(raw_spinlock_t *lock)
{
	unsigned char oldval = 1;

	asm volatile("xchgb %b0, %1"
		     : "=q" (oldval), "+m" (lock->slock)
		     : "0" (oldval) : "memory");
}

#endif

static inline void __raw_spin_unlock_wait(raw_spinlock_t *lock)
{
	while (__raw_spin_is_locked(lock))
		cpu_relax();
}

/*
 * Read-write spinlocks, allowing multiple readers
 * but only one writer.
 *
 * NOTE! it is quite common to have readers in interrupts
 * but no interrupt writers. For those circumstances we
 * can "mix" irq-safe locks - any writer needs to get a
 * irq-safe write-lock, but readers can get non-irqsafe
 * read-locks.
 *
 * On x86, we implement read-write locks as a 32-bit counter
 * with the high bit (sign) being the "contended" bit.
 */

static inline int __raw_read_can_lock(raw_rwlock_t *lock)
{
	return (int)(lock)->lock > 0;
}

static inline int __raw_write_can_lock(raw_rwlock_t *lock)
{
	return (lock)->lock == RW_LOCK_BIAS;
}

static inline void __raw_read_lock(raw_rwlock_t *rw)
{
	asm volatile(LOCK_PREFIX " subl $1,(%0)\n\t"
		     "jns 1f\n"
		     "call __read_lock_failed\n\t"
		     "1:\n"
		     ::LOCK_PTR_REG (rw) : "memory");
}

static inline void __raw_write_lock(raw_rwlock_t *rw)
{
	asm volatile(LOCK_PREFIX " subl %1,(%0)\n\t"
		     "jz 1f\n"
		     "call __write_lock_failed\n\t"
		     "1:\n"
		     ::LOCK_PTR_REG (rw), "i" (RW_LOCK_BIAS) : "memory");
}

static inline int __raw_read_trylock(raw_rwlock_t *lock)
{
	atomic_t *count = (atomic_t *)lock;

	atomic_dec(count);
	if (atomic_read(count) >= 0)
		return 1;
	atomic_inc(count);
	return 0;
}

static inline int __raw_write_trylock(raw_rwlock_t *lock)
{
	atomic_t *count = (atomic_t *)lock;

	if (atomic_sub_and_test(RW_LOCK_BIAS, count))
		return 1;
	atomic_add(RW_LOCK_BIAS, count);
	return 0;
}

static inline void __raw_read_unlock(raw_rwlock_t *rw)
{
	asm volatile(LOCK_PREFIX "incl %0" :"+m" (rw->lock) : : "memory");
}

static inline void __raw_write_unlock(raw_rwlock_t *rw)
{
	asm volatile(LOCK_PREFIX "addl %1, %0"
		     : "+m" (rw->lock) : "i" (RW_LOCK_BIAS) : "memory");
}

#define _raw_spin_relax(lock)	cpu_relax()
#define _raw_read_relax(lock)	cpu_relax()
#define _raw_write_relax(lock)	cpu_relax()

#endif