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

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> 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.

AFAIU, __thread information gets reclaimed when a thread exits, so I
don't think there is any real-life difference in our case: usually the
rcu_unregister will be called just before thread exit.

So I agree with you that __thread should provide us better performance
in the most frequent paths, so I'm in favor of leaving __thread whenever
it's available.

Thanks,

Mathieu

> 
> 							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
> > 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com