[lttng-dev] [RFC PATCH urcu] Implement urcu/tls-compat.h

[Paul E. McKenney]

On Wed, May 16, 2012 at 09:56:58AM -0400, Mathieu Desnoyers wrote:
> Suggested-by: Marek Vavruša <marek.vavrusa at nic.cz>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

Interesting!  Jeffrey Yasskin of Google was suggesting use of
pthread_get_specific() over __thread even where __thread was available.
His justification is that pthread_get_specific() allows destructors.
Not sure I agree with him in the case of userspace RCU due to performance
issues, but thought I would pass his advice along.

							Thanx, Paul

> ---
> diff --git a/Makefile.am b/Makefile.am
> index 0a369fd..6263057 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -18,7 +18,8 @@ nobase_dist_include_HEADERS = urcu/compiler.h urcu/hlist.h urcu/list.h \
>  		urcu/ref.h urcu/cds.h urcu/urcu_ref.h urcu/urcu-futex.h \
>  		urcu/uatomic_arch.h urcu/rculfhash.h \
>  		$(top_srcdir)/urcu/map/*.h \
> -		$(top_srcdir)/urcu/static/*.h
> +		$(top_srcdir)/urcu/static/*.h \
> +		urcu/tls-compat.h
>  nobase_nodist_include_HEADERS = urcu/arch.h urcu/uatomic.h urcu/config.h
>  
>  EXTRA_DIST = $(top_srcdir)/urcu/arch/*.h $(top_srcdir)/urcu/uatomic/*.h \
> diff --git a/urcu/tls-compat.h b/urcu/tls-compat.h
> new file mode 100644
> index 0000000..d7c7537
> --- /dev/null
> +++ b/urcu/tls-compat.h
> @@ -0,0 +1,99 @@
> +#ifndef _URCU_TLS_COMPAT_H
> +#define _URCU_TLS_COMPAT_H
> +
> +/*
> + * urcu/tls-compat.h
> + *
> + * Userspace RCU library - Thread-Local Storage Compatibility Header
> + *
> + * Copyright 2012 - Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> + *
> + * This library is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * This library is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with this library; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include <stdlib.h>
> +#include <urcu/config.h>
> +#include <urcu/compiler.h>
> +#include <urcu/arch.h>
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#ifdef CONFIG_RCU_TLS	/* Based on ax_tls.m4 */
> +
> +# define DECLARE_URCU_TLS(type, name)	\
> +	CONFIG_RCU_TLS type __tls_ ## name
> +
> +# define DEFINE_URCU_TLS(type, name)	\
> +	CONFIG_RCU_TLS type __tls_ ## name
> +
> +# define URCU_TLS(name)		(__tls_ ## name)
> +
> +#else /* #ifndef CONFIG_RCU_TLS */
> +
> +# include <pthread.h>
> +
> +struct urcu_tls {
> +	pthread_key_t key;
> +	pthread_mutex_t init_mutex;
> +	int init_done;
> +};
> +
> +# define DECLARE_URCU_TLS(type, name)				\
> +	type *__tls_access_ ## name(void)
> +
> +/*
> + * Note: we don't free memory at process exit, since it will be dealt
> + * with by the OS.
> + */
> +# define DEFINE_URCU_TLS(type, name)				\
> +	type *__tls_access_ ## name(void)			\
> +	{							\
> +		static struct urcu_tls __tls_ ## name = {	\
> +			.init_mutex = PTHREAD_MUTEX_INITIALIZER,\
> +			.init_done = 0,				\
> +		};						\
> +		void *__tls_p;					\
> +		if (!__tls_ ## name.init_done) {		\
> +			/* Mutex to protect concurrent init */	\
> +			pthread_mutex_lock(&__tls_ ## name.init_mutex); \
> +			if (!__tls_ ## name.init_done) {	\
> +				(void) pthread_key_create(&__tls_ ## name.key, \
> +					free);			\
> +				cmm_smp_wmb();	/* create key before write init_done */ \
> +				__tls_ ## name.init_done = 1;	\
> +			}					\
> +			pthread_mutex_unlock(&__tls_ ## name.init_mutex); \
> +		}						\
> +		cmm_smp_rmb();	/* read init_done before getting key */ \
> +		__tls_p = pthread_getspecific(__tls_ ## name.key); \
> +		if (caa_unlikely(__tls_p == NULL)) {		\
> +			__tls_p = calloc(1, sizeof(type));	\
> +			(void) pthread_setspecific(__tls_ ## name.key,	\
> +				__tls_p);			\
> +		}						\
> +		return __tls_p;					\
> +	}
> +
> +# define URCU_TLS(name)		(*__tls_access_ ## name())
> +
> +#endif	/* #else #ifndef CONFIG_RCU_TLS */
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _URCU_TLS_COMPAT_H */
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>