arch/x86/include/asm/rwsem.h - linux - Git at Google

 /* 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 */
	/* 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 */