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/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+

 *

 * Written by David Howells (dhowells@redhat.com).

 *

 * Derived from asm-x86/semaphore.h

 *

 *

 * The MSW of the count is the negated number of active writers and waiting

 * lockers, and the LSW is the total number of active locks

 *

 * The lock count is initialized to 0 (no active and no waiting lockers).

 *

 * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an

 * uncontended lock. This can be determined because XADD returns the old value.

 * Readers increment by 1 and see a positive value when uncontended, negative

 * if there are writers (and maybe) readers waiting (in which case it goes to

 * sleep).

 *

 * The value of WAITING_BIAS supports up to 32766 waiting processes. This can

 * be extended to 65534 by manually checking the whole MSW rather than relying

 * on the S flag.

 *

 * The value of ACTIVE_BIAS supports up to 65535 active processes.

 *

 * This should be totally fair - if anything is waiting, a process that wants a

 * lock will go to the back of the queue. When the currently active lock is

 * released, if there's a writer at the front of the queue, then that and only

 * that will be woken up; if there's a bunch of consequtive readers at the

 * front, then they'll all be woken up, but no other readers will be.

 */

#ifndef _ASM_X86_RWSEM_H

#define _ASM_X86_RWSEM_H

#ifndef _LINUX_RWSEM_H

#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"

#endif

#ifdef __KERNEL__

#include 

/*

 * The bias values and the counter type limits the number of

 * potential readers/writers to 32767 for 32 bits and 2147483647

 * for 64 bits.

 */

#ifdef CONFIG_X86_64

# define RWSEM_ACTIVE_MASK		0xffffffffL

#else

# define RWSEM_ACTIVE_MASK		0x0000ffffL

#endif

#define RWSEM_UNLOCKED_VALUE		0x00000000L

#define RWSEM_ACTIVE_BIAS		0x00000001L

#define RWSEM_WAITING_BIAS		(-RWSEM_ACTIVE_MASK-1)

#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS

#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)

/*

 * lock for reading

 */

static inline void __down_read(struct rw_semaphore *sem)

{

	asm volatile("# beginning down_read\n\t"

		     LOCK_PREFIX _ASM_INC "(%1)\n\t"

		     /* adds 0x00000001 */

		     "  jns        1f\n"

		     "  call call_rwsem_down_read_failed\n"

		     "1:\n\t"

		     "# ending down_read\n\t"

		     : "+m" (sem->count)

		     : "a" (sem)

		     : "memory", "cc");

}

/*

 * trylock for reading -- returns 1 if successful, 0 if contention

 */

static inline int __down_read_trylock(struct rw_semaphore *sem)

{

	long result, tmp;

	asm volatile("# beginning __down_read_trylock\n\t"

		     "  mov          %0,%1\n\t"

		     "1:\n\t"

		     "  mov          %1,%2\n\t"

		     "  add          %3,%2\n\t"

		     "  jle	     2f\n\t"

		     LOCK_PREFIX "  cmpxchg  %2,%0\n\t"

		     "  jnz	     1b\n\t"

		     "2:\n\t"

		     "# ending __down_read_trylock\n\t"

		     : "+m" (sem->count), "=&a" (result), "=&r" (tmp)

		     : "i" (RWSEM_ACTIVE_READ_BIAS)

		     : "memory", "cc");

	return result >= 0 ? 1 : 0;

}

/*

 * lock for writing

 */

static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)

{

	long tmp;

	asm volatile("# beginning down_write\n\t"

		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"

		     /* adds 0xffff0001, returns the old value */

		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"

		     /* was the active mask 0 before? */

		     "  jz        1f\n"

		     "  call call_rwsem_down_write_failed\n"

		     "1:\n"

		     "# ending down_write"

		     : "+m" (sem->count), "=d" (tmp)

		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)

		     : "memory", "cc");

}

static inline void __down_write(struct rw_semaphore *sem)

{

	__down_write_nested(sem, 0);

}

/*

 * trylock for writing -- returns 1 if successful, 0 if contention

 */

static inline int __down_write_trylock(struct rw_semaphore *sem)

{

	long result, tmp;

	asm volatile("# beginning __down_write_trylock\n\t"

		     "  mov          %0,%1\n\t"

		     "1:\n\t"

		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"

		     /* was the active mask 0 before? */

		     "  jnz          2f\n\t"

		     "  mov          %1,%2\n\t"

		     "  add          %3,%2\n\t"

		     LOCK_PREFIX "  cmpxchg  %2,%0\n\t"

		     "  jnz	     1b\n\t"

		     "2:\n\t"

		     "  sete         %b1\n\t"

		     "  movzbl       %b1, %k1\n\t"

		     "# ending __down_write_trylock\n\t"

		     : "+m" (sem->count), "=&a" (result), "=&r" (tmp)

		     : "er" (RWSEM_ACTIVE_WRITE_BIAS)

		     : "memory", "cc");

	return result;

}

/*

 * unlock after reading

 */

static inline void __up_read(struct rw_semaphore *sem)

{

	long tmp;

	asm volatile("# beginning __up_read\n\t"

		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"

		     /* subtracts 1, returns the old value */

		     "  jns        1f\n\t"

		     "  call call_rwsem_wake\n" /* expects old value in %edx */

		     "1:\n"

		     "# ending __up_read\n"

		     : "+m" (sem->count), "=d" (tmp)

		     : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)

		     : "memory", "cc");

}

/*

 * unlock after writing

 */

static inline void __up_write(struct rw_semaphore *sem)

{

	long tmp;

	asm volatile("# beginning __up_write\n\t"

		     LOCK_PREFIX "  xadd      %1,(%2)\n\t"

		     /* subtracts 0xffff0001, returns the old value */

		     "  jns        1f\n\t"

		     "  call call_rwsem_wake\n" /* expects old value in %edx */

		     "1:\n\t"

		     "# ending __up_write\n"

		     : "+m" (sem->count), "=d" (tmp)

		     : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)

		     : "memory", "cc");

}

/*

 * downgrade write lock to read lock

 */

static inline void __downgrade_write(struct rw_semaphore *sem)

{

	asm volatile("# beginning __downgrade_write\n\t"

		     LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"

		     /*

		      * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)

		      *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)

		      */

		     "  jns       1f\n\t"

		     "  call call_rwsem_downgrade_wake\n"

		     "1:\n\t"

		     "# ending __downgrade_write\n"

		     : "+m" (sem->count)

		     : "a" (sem), "er" (-RWSEM_WAITING_BIAS)

		     : "memory", "cc");

}

/*

 * implement atomic add functionality

 */

static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)

{

	asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0"

		     : "+m" (sem->count)

		     : "er" (delta));

}

/*

 * implement exchange and add functionality

 */

static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)

{

	return delta + xadd(&sem->count, delta);

}

#endif /* __KERNEL__ */

#endif /* _ASM_X86_RWSEM_H */