arch/x86/include/asm/rwsem.h

/* SPDX-License-Identifier: GPL-2.0 */
/* 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 consecutive 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 <asm/asm.h>

/*
 * 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
 */
#define ____down_read(sem, slow_path)					\
({									\
	struct rw_semaphore* ret;					\
	asm volatile("# beginning down_read\n\t"			\
		     LOCK_PREFIX _ASM_INC "(%[sem])\n\t"		\
		     /* adds 0x00000001 */				\
		     "  jns        1f\n"				\
		     "  call " slow_path "\n"				\
		     "1:\n\t"						\
		     "# ending down_read\n\t"				\
		     : "+m" (sem->count), "=a" (ret),			\
			ASM_CALL_CONSTRAINT				\
		     : [sem] "a" (sem)					\
		     : "memory", "cc");					\
	ret;								\
})

static inline void __down_read(struct rw_semaphore *sem)
{
	____down_read(sem, "call_rwsem_down_read_failed");
}

static inline int __down_read_killable(struct rw_semaphore *sem)
{
	if (IS_ERR(____down_read(sem, "call_rwsem_down_read_failed_killable")))
		return -EINTR;
	return 0;
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
static inline bool __down_read_trylock(struct rw_semaphore *sem)
{
	long result, tmp;
	asm volatile("# beginning __down_read_trylock\n\t"
		     "  mov          %[count],%[result]\n\t"
		     "1:\n\t"
		     "  mov          %[result],%[tmp]\n\t"
		     "  add          %[inc],%[tmp]\n\t"
		     "  jle	     2f\n\t"
		     LOCK_PREFIX "  cmpxchg  %[tmp],%[count]\n\t"
		     "  jnz	     1b\n\t"
		     "2:\n\t"
		     "# ending __down_read_trylock\n\t"
		     : [count] "+m" (sem->count), [result] "=&a" (result),
		       [tmp] "=&r" (tmp)
		     : [inc] "i" (RWSEM_ACTIVE_READ_BIAS)
		     : "memory", "cc");
	return result >= 0;
}

/*
 * lock for writing
 */
#define ____down_write(sem, slow_path)			\
({							\
	long tmp;					\
	struct rw_semaphore* ret;			\
							\
	asm volatile("# beginning down_write\n\t"	\
		     LOCK_PREFIX "  xadd      %[tmp],(%[sem])\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 " slow_path "\n"		\
		     "1:\n"				\
		     "# ending down_write"		\
		     : "+m" (sem->count), [tmp] "=d" (tmp),	\
		       "=a" (ret), ASM_CALL_CONSTRAINT	\
		     : [sem] "a" (sem), "[tmp]" (RWSEM_ACTIVE_WRITE_BIAS) \
		     : "memory", "cc");			\
	ret;						\
})

static inline void __down_write(struct rw_semaphore *sem)
{
	____down_write(sem, "call_rwsem_down_write_failed");
}

static inline int __down_write_killable(struct rw_semaphore *sem)
{
	if (IS_ERR(____down_write(sem, "call_rwsem_down_write_failed_killable")))
		return -EINTR;

	return 0;
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
static inline bool __down_write_trylock(struct rw_semaphore *sem)
{
	bool result;
	long tmp0, tmp1;
	asm volatile("# beginning __down_write_trylock\n\t"
		     "  mov          %[count],%[tmp0]\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          %[tmp0],%[tmp1]\n\t"
		     "  add          %[inc],%[tmp1]\n\t"
		     LOCK_PREFIX "  cmpxchg  %[tmp1],%[count]\n\t"
		     "  jnz	     1b\n\t"
		     "2:\n\t"
		     CC_SET(e)
		     "# ending __down_write_trylock\n\t"
		     : [count] "+m" (sem->count), [tmp0] "=&a" (tmp0),
		       [tmp1] "=&r" (tmp1), CC_OUT(e) (result)
		     : [inc] "er" (RWSEM_ACTIVE_WRITE_BIAS)
		     : "memory");
	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      %[tmp],(%[sem])\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), [tmp] "=d" (tmp)
		     : [sem] "a" (sem), "[tmp]" (-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      %[tmp],(%[sem])\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), [tmp] "=d" (tmp)
		     : [sem] "a" (sem), "[tmp]" (-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 "%[inc],(%[sem])\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)
		     : [sem] "a" (sem), [inc] "er" (-RWSEM_WAITING_BIAS)
		     : "memory", "cc");
}

#endif /* __KERNEL__ */
#endif /* _ASM_X86_RWSEM_H */