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SubjectRe: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
David Howells <dhowells@redhat.com> wrote:
>
> The attached patch introduces a simple mutex implementation as an alternative
> to the usual semaphore implementation where simple mutex functionality is all
> that is required.
>
> This is useful in two ways:
>
> (1) A number of archs only provide very simple atomic instructions (such as
> XCHG on i386, TAS on M68K, SWAP on FRV) which aren't sufficient to
> implement full semaphore support directly. Instead spinlocks must be
> employed to implement fuller functionality.
>
> (2) This makes it obvious in what way the semaphore is being used: whether
> it's being used as a mutex or being used as a counter.
>
> This patch set does the following:
>
> (1) Provides a simple xchg() based semaphore as a default for all
> architectures that don't wish to override it and provide their own.
>
> Overriding is possible by setting CONFIG_ARCH_IMPLEMENTS_MUTEX and
> supplying asm/mutex.h
>
> Partial overriding is possible by #defining mutex_grab(), mutex_release()
> and is_mutex_locked() to perform the appropriate optimised functions.
>
> (2) Provides linux/mutex.h as a common include for gaining access to mutex
> semaphores.
>
> (3) Provides linux/semaphore.h as a common include for gaining access to all
> the different types of semaphore that may be used from within the kernel.
>
> (4) Renames down*() to down_sem*() and up() to up_sem() for the traditional
> semaphores, and removes init_MUTEX*() and DECLARE_MUTEX*() from
> asm/semaphore.h
>
> (5) Redirects the following to apply to the new mutexes rather than the
> traditional semaphores:
>
> down()
> down_trylock()
> down_interruptible()
> up()
> init_MUTEX()
> init_MUTEX_LOCKED()
> DECLARE_MUTEX()
> DECLARE_MUTEX_LOCKED()
>
> On the basis that most usages of semaphores are as mutexes, this makes
> sense for in most cases it's just then a matter of changing the type from
> struct semaphore to struct mutex. In some cases, sema_init() has to be
> changed to init_MUTEX*() also.
>
> (6) Generally include linux/semaphore.h in place of asm/semaphore.h.
>
> (7) Provides a debugging config option CONFIG_DEBUG_MUTEX_OWNER by which the
> mutex owner can be tracked and by which over-upping can be detected.

Maybe I'm not understanding all this, but...

I'd have thought that the way to do this is to simply reimplement down(),
up(), down_trylock(), etc using the new xchg-based code and to then hunt
down those few parts of the kernel which actually use the old semaphore's
counting feature and convert them to use down_sem(), up_sem(), etc. And
rename all the old semaphore code: s/down/down_sem/etc.

So after such a transformation, this new "mutex" thingy would not exist.

> include/linux/mutex.h | 32 +++++++

But it does.

> +#define mutex_grab(mutex) (xchg(&(mutex)->state, 1) == 0)

mutex_trylock(), please.

> +#define is_mutex_locked(mutex) ((mutex)->state)

Let's keep the namespace consistent. mutex_is_locked().

> +static inline void down(struct mutex *mutex)
> +{
> + if (mutex_grab(mutex)) {

likely()

> +#ifdef CONFIG_DEBUG_MUTEX_OWNER
> + mutex->__owner = current;
> +#endif
> + }
> + else {
> + __down(mutex);
> + }
> +}
> +
> +/*
> + * sleep interruptibly until we get the mutex
> + * - return 0 if successful, -EINTR if interrupted
> + */
> +static inline int down_interruptible(struct mutex *mutex)
> +{
> + if (mutex_grab(mutex)) {

likely()

> +static inline int down_trylock(struct mutex *mutex)
> +{
> + if (mutex_grab(mutex)) {

etc.

You could just put likely() into mutex_trylock(). err, mutex_grab().

> +/*
> + * release the mutex
> + */
> +static inline void up(struct mutex *mutex)
> +{
> + unsigned long flags;
> +
> +#ifdef CONFIG_DEBUG_MUTEX_OWNER
> + if (mutex->__owner != current)
> + __up_bad(mutex);
> + mutex->__owner = NULL;
> +#endif
> +
> + /* must prevent a race */
> + spin_lock_irqsave(&mutex->wait_lock, flags);
> + if (!list_empty(&mutex->wait_list))
> + __up(mutex);
> + else
> + mutex_release(mutex);
> + spin_unlock_irqrestore(&mutex->wait_lock, flags);
> +}

This is too large to inline.

It's also significantly slower than the existing up()?

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