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Re: [PATCH 0/1] trace2: fix tracing when NO_PTHREADS is defined




On Thu, May 23, 2019 at 12:51 PM Jeff King <peff@xxxxxxxx> wrote:
> For fun, here's a constant-time zero-allocation implementation that I
> came up with. It passes t0211 with NO_PTHREADS, but I didn't test it
> beyond that.
>
> diff --git a/thread-utils.h b/thread-utils.h
> index 4961487ed9..f466215742 100644
> --- a/thread-utils.h
> +++ b/thread-utils.h
> @@ -18,7 +18,7 @@
>  #define pthread_t int
>  #define pthread_mutex_t int
>  #define pthread_cond_t int
> -#define pthread_key_t int
> +#define pthread_key_t git_pthread_key_t
>
>  #define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
>  #define pthread_mutex_lock(mutex)
> @@ -31,16 +31,49 @@
>  #define pthread_cond_broadcast(cond)
>  #define pthread_cond_destroy(cond)
>
> -#define pthread_key_create(key, attr) dummy_pthread_init(key)
> -#define pthread_key_delete(key)
> +#define pthread_key_create(key, destroy) git_pthread_key_create(key, destroy)
> +#define pthread_key_delete(key) git_pthread_key_delete(key)
>
>  #define pthread_create(thread, attr, fn, data) \
>         dummy_pthread_create(thread, attr, fn, data)
>  #define pthread_join(thread, retval) \
>         dummy_pthread_join(thread, retval)
>
> -#define pthread_setspecific(key, data)
> -#define pthread_getspecific(key) NULL
> +#define pthread_setspecific(key, data) git_pthread_setspecific(key, data)
> +#define pthread_getspecific(key) git_pthread_getspecific(key)
> +
> +typedef struct {
> +       void *data;
> +       /* extra indirection because setspecific is passed key by value */
> +       void **vdata;

Ha! I was thinking a separate key->value mapping which is complicated
in C. But this works pretty well for a single thread, and it even
supports multiple keys.

> +} git_pthread_key_t;
> +
> +static inline int git_pthread_key_create(git_pthread_key_t *key,
> +                                        void (*destroy)(void *))
> +{
> +       key->data = NULL;
> +       key->vdata = &key->data;
> +       /* We don't use this; alternatively we could all via atexit(). */
> +       if (destroy)
> +               BUG("NO_PTHREADS does not support pthread key destructors");
> +       return 0;
> +}
> +
> +static inline int git_pthread_key_delete(git_pthread_key_t key)
> +{
> +       /* noop */
> +       return 0;
> +}
> +
> +static inline void git_pthread_setspecific(git_pthread_key_t key, void *data)
> +{
> +       *(key.vdata) = data;
> +}
> +
> +static inline void *git_pthread_getspecific(git_pthread_key_t key)
> +{
> +       return key.data;
> +}
>
>  int dummy_pthread_create(pthread_t *pthread, const void *attr,
>                          void *(*fn)(void *), void *data);
-- 
Duy