lkml.org 
[lkml]   [2005]   [Dec]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: [patch 00/11] mutex subsystem, -V7
    On Fri, 23 Dec 2005, Ingo Molnar wrote:

    > this is version -V7 of the generic mutex subsystem. It consists of the
    > following 11 patches:

    Here's another one, allowing architecture specific trylock
    implementations. New generic xchg-based and ARMv6 implementations are
    provided, while everything else remains the same.

    Signed-off-by: Nicolas Pitre <nico@cam.org>

    Index: linux-2.6/include/asm-generic/mutex-xchg.h
    ===================================================================
    --- linux-2.6.orig/include/asm-generic/mutex-xchg.h
    +++ linux-2.6/include/asm-generic/mutex-xchg.h
    @@ -69,4 +69,38 @@ do { \

    #define __mutex_slowpath_needs_to_unlock() 0

    +/**
    + * __mutex_trylock - try to acquire the mutex, without waiting
    + *
    + * @count: pointer of type atomic_t
    + *
    + * 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.
    + */
    +static inline int
    +__mutex_trylock(atomic_t *count)
    +{
    + 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 all cases this has the potential to trigger the
    + * slowpath for the owner's unlock path - but this is not
    + * a big problem in practice.
    + */
    + prev = atomic_xchg(count, -1);
    + if (prev < 0)
    + prev = 0;
    + }
    +
    + return prev;
    +}
    +
    #endif
    Index: linux-2.6/include/asm-i386/mutex.h
    ===================================================================
    --- linux-2.6.orig/include/asm-i386/mutex.h
    +++ linux-2.6/include/asm-i386/mutex.h
    @@ -99,4 +99,38 @@ do { \

    #define __mutex_slowpath_needs_to_unlock() 1

    +/**
    + * __mutex_trylock - try to acquire the mutex, without waiting
    + *
    + * @count: pointer of type atomic_t
    + *
    + * 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.
    + */
    +static inline int
    +__mutex_trylock(atomic_t *count)
    +{
    + /*
    + * We have two variants here. The cmpxchg based one is the best one
    + * because it never induce a false contention state.
    + * If not available we fall back to the atomic_dec_return variant.
    + *
    + * The atomic_dec_return variant might end up making the counter
    + * negative in the failure case, which may trigger the slowpath
    + * for the owner's unlock path - but this is not a big problem
    + * in practice.
    + */
    +#ifdef __HAVE_ARCH_CMPXCHG
    + if (likely(atomic_cmpxchg(count, 1, 0)) == 1)
    + return 1;
    +#else
    + if (likely(atomic_dec_return(count) == 0))
    + return 1;
    +#endif
    + return 0;
    +}
    +
    #endif
    Index: linux-2.6/include/asm-x86_64/mutex.h
    ===================================================================
    --- linux-2.6.orig/include/asm-x86_64/mutex.h
    +++ linux-2.6/include/asm-x86_64/mutex.h
    @@ -71,4 +71,21 @@ do { \

    #define __mutex_slowpath_needs_to_unlock() 1

    +/**
    + * __mutex_trylock - try to acquire the mutex, without waiting
    + *
    + * @count: pointer of type atomic_t
    + *
    + * Change the count from 1 to 0 and return 1 (success), or return 0 (failure)
    + * if it wasn't 1 originally.
    + */
    +static inline int
    +__mutex_trylock(atomic_t *count)
    +{
    + if (likely(atomic_cmpxchg(count, 1, 0)) == 1)
    + return 1;
    + else
    + return 0;
    +}
    +
    #endif
    Index: linux-2.6/kernel/mutex.c
    ===================================================================
    --- linux-2.6.orig/kernel/mutex.c
    +++ linux-2.6/kernel/mutex.c
    @@ -219,19 +219,6 @@ __mutex_lock_interruptible_nonatomic(str
    return -EINTR;
    }

    -/*
    - * We have three mutex_trylock() variants. The cmpxchg based one is
    - * the best one (because it has no side-effect on mutex_unlock()),
    - * but cmpxchg is not available on every architecture, so we also
    - * provide an atomic_dec_return based variant too. The debug variant
    - * takes the internal lock.
    - *
    - * [ The atomic_dec_return variant might end up making the counter
    - * negative in the failure case, which may trigger the slowpath
    - * for the owner's unlock path - but this is not a big problem
    - * in practice. ]
    - */
    -#ifndef CONFIG_DEBUG_MUTEXES
    /***
    * mutex_trylock - try acquire the mutex, without waiting
    * @lock: the mutex to be acquired
    @@ -248,24 +235,13 @@ __mutex_lock_interruptible_nonatomic(str
    */
    int fastcall mutex_trylock(struct mutex *lock)
    {
    -#ifdef __HAVE_ARCH_CMPXCHG
    - if (atomic_cmpxchg(&lock->count, 1, 0) == 1)
    - return 1;
    +#ifndef CONFIG_DEBUG_MUTEXES
    + return __mutex_trylock(&lock->count);
    #else
    - if (atomic_dec_return(&lock->count) == 0)
    - return 1;
    -#endif
    - return 0;
    -}
    -
    -#else /* CONFIG_DEBUG_MUTEXES: */
    -
    -/*
    - * In the debug case we take the spinlock and check whether we can
    - * get the lock:
    - */
    -int fastcall mutex_trylock(struct mutex *lock)
    -{
    + /*
    + * In the debug case we take the spinlock and check whether we can
    + * get the lock:
    + */
    struct thread_info *ti = current_thread_info();
    int ret = 0;

    @@ -280,10 +256,9 @@ int fastcall mutex_trylock(struct mutex
    spin_unlock_mutex(&lock->wait_lock);

    return ret;
    +#endif
    }

    -#endif /* CONFIG_DEBUG_MUTEXES */
    -
    /*
    * Release the lock, slowpath:
    */
    Index: linux-2.6/include/asm-arm/mutex.h
    ===================================================================
    --- linux-2.6.orig/include/asm-arm/mutex.h
    +++ linux-2.6/include/asm-arm/mutex.h
    @@ -98,5 +98,31 @@ do { \
    */
    #define __mutex_slowpath_needs_to_unlock() 1

    +/*
    + * For __mutex_trylock we use another construct which could be described
    + * as an "incomplete atomic decrement" or a "single value cmpxchg" since
    + * it has two modes of failure:
    + *
    + * 1) if the exclusive store fails we fail, and
    + *
    + * 2) if the decremented value is not zero we don't even attempt the store.
    + *
    + * This provides the needed trylock semantics like cmpxchg would, but it is
    + * lighter and less generic than a true cmpxchg implementation.
    + */
    +static inline int __mutex_trylock(atomic_t *count)
    +{
    + int __ex_flag, __res;
    + __asm__ (
    + "ldrex %0, [%2]\n\t"
    + "subs %0, %0, #1\n\t"
    + "strexeq %1, %0, [%2]"
    + : "=&r" (__res), "=&r" (__ex_flag)
    + : "r" (&count->counter)
    + : "cc","memory" );
    + __res |= __ex_flag;
    + return __res == 0;
    +}
    +
    #endif
    #endif
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/
    \
     
     \ /
      Last update: 2005-12-24 06:25    [W:0.061 / U:29.216 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site