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Commit 620a6fd1 authored by Ingo Molnar's avatar Ingo Molnar Committed by Ingo Molnar
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[PATCH] mutex subsystem, add asm-generic/mutex-[dec|xchg|null].h implementations 



Add three (generic) mutex fastpath implementations.

The mutex-xchg.h implementation is atomic_xchg() based, and should
work fine on every architecture.

The mutex-dec.h implementation is atomic_dec_return() based - this
one too should work on every architecture, but might not perform the
most optimally on architectures that have no atomic-dec/inc instructions.

The mutex-null.h implementation forces all calls into the slowpath. This
is used for mutex debugging, but it can also be used on platforms that do
not want (or need) a fastpath at all.

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarArjan van de Ven <arjan@infradead.org>
parent 711a660d

include/asm-generic/mutex-dec.h

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/*

 * asm-generic/mutex-dec.h

 *

 * Generic implementation of the mutex fastpath, based on atomic

 * decrement/increment.

 */

#ifndef _ASM_GENERIC_MUTEX_DEC_H

#define _ASM_GENERIC_MUTEX_DEC_H



/**

 *  __mutex_fastpath_lock - try to take the lock by moving the count

 *                          from 1 to a 0 value

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 1

 *

 * Change the count from 1 to a value lower than 1, and call <fail_fn> if

 * it wasn't 1 originally. This function MUST leave the value lower than

 * 1 even when the "1" assertion wasn't true.

 */

#define __mutex_fastpath_lock(count, fail_fn)				\

do {									\

	if (unlikely(atomic_dec_return(count) < 0))			\

		fail_fn(count);						\

	else								\

		smp_mb();						\

} while (0)



/**

 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count

 *                                 from 1 to a 0 value

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 1

 *

 * Change the count from 1 to a value lower than 1, and call <fail_fn> if

 * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,

 * or anything the slow path function returns.

 */

static inline int

__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))

{

	if (unlikely(atomic_dec_return(count) < 0))

		return fail_fn(count);

	else {

		smp_mb();

		return 0;

	}

}



/**

 *  __mutex_fastpath_unlock - try to promote the count from 0 to 1

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 0

 *

 * Try to promote the count from 0 to 1. If it wasn't 0, call <fail_fn>.

 * In the failure case, this function is allowed to either set the value to

 * 1, or to set it to a value lower than 1.

 *

 * If the implementation sets it to a value of lower than 1, then the

 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs

 * to return 0 otherwise.

 */

#define __mutex_fastpath_unlock(count, fail_fn)				\

do {									\

	smp_mb();							\

	if (unlikely(atomic_inc_return(count) <= 0))			\

		fail_fn(count);						\

} while (0)



#define __mutex_slowpath_needs_to_unlock()		1



/**

 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting

 *

 *  @count: pointer of type atomic_t

 *  @fail_fn: fallback function

 *

 * Change the count from 1 to a value lower than 1, and return 0 (failure)

 * if it wasn't 1 originally, or return 1 (success) otherwise. This function

 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.

 * Additionally, if the value was < 0 originally, this function must not leave

 * it to 0 on failure.

 *

 * If the architecture has no effective trylock variant, it should call the

 * <fail_fn> spinlock-based trylock variant unconditionally.

 */

static inline int

__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))

{

	/*

	 * We have two variants here. The cmpxchg based one is the best one

	 * because it never induce a false contention state.  It is included

	 * here because architectures using the inc/dec algorithms over the

	 * xchg ones are much more likely to support cmpxchg natively.

	 *

	 * If not we fall back to the spinlock based variant - that is

	 * just as efficient (and simpler) as a 'destructive' probing of

	 * the mutex state would be.

	 */

#ifdef __HAVE_ARCH_CMPXCHG

	if (likely(atomic_cmpxchg(count, 1, 0)) == 1) {

		smp_mb();

		return 1;

	}

	return 0;

#else

	return fail_fn(count);

#endif

}



#endif

include/asm-generic/mutex-null.h

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/*

 * asm-generic/mutex-null.h

 *

 * Generic implementation of the mutex fastpath, based on NOP :-)

 *

 * This is used by the mutex-debugging infrastructure, but it can also

 * be used by architectures that (for whatever reason) want to use the

 * spinlock based slowpath.

 */

#ifndef _ASM_GENERIC_MUTEX_NULL_H

#define _ASM_GENERIC_MUTEX_NULL_H



/* extra parameter only needed for mutex debugging: */

#ifndef __IP__

# define __IP__

#endif



#define __mutex_fastpath_lock(count, fail_fn)	      fail_fn(count __RET_IP__)

#define __mutex_fastpath_lock_retval(count, fail_fn)  fail_fn(count __RET_IP__)

#define __mutex_fastpath_unlock(count, fail_fn)       fail_fn(count __RET_IP__)

#define __mutex_fastpath_trylock(count, fail_fn)      fail_fn(count)

#define __mutex_slowpath_needs_to_unlock()	      1



#endif

include/asm-generic/mutex-xchg.h

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/*

 * asm-generic/mutex-xchg.h

 *

 * Generic implementation of the mutex fastpath, based on xchg().

 *

 * NOTE: An xchg based implementation is less optimal than an atomic

 *       decrement/increment based implementation. If your architecture

 *       has a reasonable atomic dec/inc then you should probably use

 *	 asm-generic/mutex-dec.h instead, or you could open-code an

 *	 optimized version in asm/mutex.h.

 */

#ifndef _ASM_GENERIC_MUTEX_XCHG_H

#define _ASM_GENERIC_MUTEX_XCHG_H



/**

 *  __mutex_fastpath_lock - try to take the lock by moving the count

 *                          from 1 to a 0 value

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 1

 *

 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it

 * wasn't 1 originally. This function MUST leave the value lower than 1

 * even when the "1" assertion wasn't true.

 */

#define __mutex_fastpath_lock(count, fail_fn)				\

do {									\

	if (unlikely(atomic_xchg(count, 0) != 1))			\

		fail_fn(count);						\

	else								\

		smp_mb();						\

} while (0)





/**

 *  __mutex_fastpath_lock_retval - try to take the lock by moving the count

 *                                 from 1 to a 0 value

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 1

 *

 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it

 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,

 * or anything the slow path function returns

 */

static inline int

__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))

{

	if (unlikely(atomic_xchg(count, 0) != 1))

		return fail_fn(count);

	else {

		smp_mb();

		return 0;

	}

}



/**

 *  __mutex_fastpath_unlock - try to promote the mutex from 0 to 1

 *  @count: pointer of type atomic_t

 *  @fail_fn: function to call if the original value was not 0

 *

 * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>

 * In the failure case, this function is allowed to either set the value to

 * 1, or to set it to a value lower than one.

 * If the implementation sets it to a value of lower than one, the

 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs

 * to return 0 otherwise.

 */

#define __mutex_fastpath_unlock(count, fail_fn)				\

do {									\

	smp_mb();							\

	if (unlikely(atomic_xchg(count, 1) != 0))			\

		fail_fn(count);						\

} while (0)



#define __mutex_slowpath_needs_to_unlock()		0



/**

 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting

 *

 *  @count: pointer of type atomic_t

 *  @fail_fn: spinlock based trylock implementation

 *

 * Change the count from 1 to a value lower than 1, and return 0 (failure)

 * if it wasn't 1 originally, or return 1 (success) otherwise. This function

 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.

 * Additionally, if the value was < 0 originally, this function must not leave

 * it to 0 on failure.

 *

 * If the architecture has no effective trylock variant, it should call the

 * <fail_fn> spinlock-based trylock variant unconditionally.

 */

static inline int

__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))

{

	int prev = atomic_xchg(count, 0);



	if (unlikely(prev < 0)) {

		/*

		 * The lock was marked contended so we must restore that

		 * state. If while doing so we get back a prev value of 1

		 * then we just own it.

		 *

		 * [ In the rare case of the mutex going to 1, to 0, to -1

		 *   and then back to 0 in this few-instructions window,

		 *   this has the potential to trigger the slowpath for the

		 *   owner's unlock path needlessly, but that's not a problem

		 *   in practice. ]

		 */

		prev = atomic_xchg(count, prev);

		if (prev < 0)

			prev = 0;

	}

	smp_mb();



	return prev;

}



#endif