Re: [PATCH 02/11] Add the low level infrastructure for pthreads lock elision with TSX

[Torvald Riegel <triegel at redhat dot com>]

On Tue, 2013-06-11 at 09:50 -0700, Andi Kleen wrote:
> Currently elision is enabled by default on systems that support RTM,
> unless explicitely disabled either in the program or by the user.

We don't really pay attention to the git comments currently AFAIK, but I
guess keeping them up-to-date can't be bad.

> diff --git a/nptl/elision-conf.h b/nptl/elision-conf.h
> new file mode 100644
> index 0000000..40a8c17
> --- /dev/null
> +++ b/nptl/elision-conf.h
> @@ -0,0 +1 @@
> +/* empty */
> diff --git a/nptl/nptl-init.c b/nptl/nptl-init.c
> index 63fb729..1cc74e6 100644
> --- a/nptl/nptl-init.c
> +++ b/nptl/nptl-init.c
> @@ -36,6 +36,8 @@
>  #include <lowlevellock.h>
>  #include <kernel-features.h>
>  
> +/* Force elision for all new locks.  */
> +int __pthread_force_elision attribute_hidden;

This really needs an extensive comment on what that does, or reference
some documentation of the overall design.  We cannot expect everyone to
have to look through all the code to have at least some idea of how that
works.  If it were intuitive, a short comment would be fine -- but it
isn't.  E.g., if it were named something like "elision_on_by_default",
then it would at least be descriptive.

Why is this defined at a different place than the rwlock flag? (which is
in elision-conf.c)?

>  
>  /* Size and alignment of static TLS block.  */
>  size_t __static_tls_size;
> diff --git a/nptl/pthreadP.h b/nptl/pthreadP.h
> index fd52b07..31cae86 100644
> --- a/nptl/pthreadP.h
> +++ b/nptl/pthreadP.h
> @@ -571,6 +571,8 @@ extern void __free_stacks (size_t limit) attribute_hidden;
>  
>  extern void __wait_lookup_done (void) attribute_hidden;
>  
> +extern int __pthread_force_elision attribute_hidden;
> +
>  #ifdef SHARED
>  # define PTHREAD_STATIC_FN_REQUIRE(name)
>  #else
> diff --git a/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h b/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
> index f665ac9..cef9588 100644
> --- a/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
> +++ b/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
> @@ -430,6 +430,12 @@ LLL_STUB_UNWIND_INFO_END
>  		       : "memory");					      \
>       result; })
>  
> +extern int __lll_timedlock_elision (int *futex, short *try_lock,
> +					 const struct timespec *timeout,
> +					 int private) attribute_hidden;
> +
> +#define lll_timedlock_elision(futex, try_lock, timeout, private)	\
> +  __lll_timedlock_elision(&(futex), &(try_lock), timeout, private)
>  
>  #define lll_robust_timedlock(futex, timeout, id, private) \
>    ({ int result, ignore1, ignore2, ignore3;				      \
> @@ -583,6 +589,22 @@ extern int __lll_timedwait_tid (int *tid, const struct timespec *abstime)
>        }									      \
>      __result; })
>  
> +extern int __lll_lock_elision (int *futex, short *try_lock, int private)
> +  attribute_hidden;
> +
> +extern int __lll_unlock_elision(int *lock, int private)
> +  attribute_hidden;
> +
> +extern int __lll_trylock_elision(int *lock, short *try_lock, int upgrade)
> +  attribute_hidden;
> +
> +#define lll_lock_elision(futex, try_lock, private) \
> +  __lll_lock_elision (&(futex), &(try_lock), private)
> +#define lll_unlock_elision(futex, private) \
> +  __lll_unlock_elision (&(futex), private)
> +#define lll_trylock_elision(futex, try_lock, upgrade) \
> +  __lll_trylock_elision(&(futex), &(try_lock), upgrade)
> +
>  #endif  /* !__ASSEMBLER__ */
>  
>  #endif	/* lowlevellock.h */
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/Makefile b/nptl/sysdeps/unix/sysv/linux/x86/Makefile
> new file mode 100644
> index 0000000..61b7552
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/Makefile
> @@ -0,0 +1,3 @@
> +libpthread-sysdep_routines += init-arch
> +libpthread-sysdep_routines += elision-lock elision-unlock elision-timed \
> +			      elision-trylock
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.c
> new file mode 100644
> index 0000000..a7f5e15
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.c
> @@ -0,0 +1,211 @@
> +/* elision-conf.c: Lock elision tunable parameters.
> +   Copyright (C) 2013 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C 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.
> +
> +   The GNU C 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 the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>. */
> +
> +#include <pthreadP.h>
> +#include <sys/fcntl.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <init-arch.h>
> +#include <elision-conf.h>
> +#include <glibc-var.h>
> +
> +struct elision_config __elision_aconf =
> +  {
> +    .skip_lock_busy = 3,
> +    .skip_lock_internal_abort = 3,
> +    .retry_try_xbegin = 3,
> +    .skip_trylock_internal_abort = 3,
> +  };
> +
> +struct tune
> +{
> +  const char *name;
> +  unsigned offset;
> +  int len;
> +};
> +
> +#define FIELD(x) { #x, offsetof(struct elision_config, x), sizeof(#x)-1 }
> +
> +static const struct tune tunings[] =
> +  {
> +    FIELD(skip_lock_busy),
> +    FIELD(skip_lock_internal_abort),
> +    FIELD(retry_try_xbegin),
> +    FIELD(skip_trylock_internal_abort),
> +    {}
> +  };
> +
> +#define PAIR(x) x, sizeof (x)-1
> +
> +/* Complain.  */
> +
> +static void
> +complain (const char *msg, int len)
> +{
> +  INTERNAL_SYSCALL_DECL (err);
> +  INTERNAL_SYSCALL (write, err, 3, 2, (char *)msg, len);
> +}
> +
> +/* Parse configuration information.  */
> +
> +static void
> +elision_aconf_setup (const char *s)
> +{
> +  int i;
> +
> +  while (*s)
> +    {
> +      for (i = 0; tunings[i].name != NULL; i++)
> +	{
> +	  int nlen = tunings[i].len;
> +
> +	  if (strncmp (tunings[i].name, s, nlen) == 0)
> +	    {
> +	      char *end;
> +	      int val;
> +
> +	      if (s[nlen] != '=')
> +		{
> +  		  complain (PAIR ("pthreads: invalid GLIBC_PTHREAD_MUTEX syntax: missing =\n"));
> +	 	  return;
> +		}
> +	      s += nlen + 1;
> +	      val = strtoul (s, &end, 0);
> +	      if (end == s)
> +		{
> +  		  complain (PAIR ("pthreads: invalid GLIBC_PTHREAD_MUTEX syntax: missing number\n"));
> +	 	  return;
> +		}
> +	      *(int *)(((char *)&__elision_aconf) + tunings[i].offset) = val;
> +	      s = end;
> +	      if (*s == ',' || *s == ':')
> +		s++;
> +	      else if (*s)
> +		{
> +  		  complain (PAIR ("pthreads: invalid GLIBC_PTHREAD_MUTEX syntax: garbage after number\n"));
> +	 	  return;
> +		}
> +	      break;
> +	    }
> +	}
> +      if (tunings[i].name == NULL)
> +      	{
> +  	  complain (PAIR ("pthreads: invalid GLIBC_PTHREAD_MUTEX syntax: unknown tunable\n"));
> + 	  return;
> +	}
> +    }
> +}
> +
> +/* Elided rwlock toggle.  */
> +
> +int __rwlock_rtm_enabled attribute_hidden;

Same here, you need to comment somewhere how that all glues together.

Why doesn't this have a name similar to __pthread_force_elision?

> +
> +/* Retries for elided rwlocks.  */
> +
> +int __rwlock_rtm_read_retries attribute_hidden = 3;
> +
> +/* Global elision check switch.  */
> +
> +int __elision_available attribute_hidden;

Improve the comment.  At least say that this is is true iff the hardware
supports elision, and it is enabled.

I'd prefer if these would be prefixed lock_elision, not just elision.
That makes the variable name longer, but doesn't collides with any other
kinds of eliding something.

> +
> +/* Initialize elision mutex.  */

s/elision mutex/lock elision for mutexes/.  Otherwise, this means
initializing the elision mutex.

> +
> +static void
> +elision_mutex_init (const char *s)
> +{
> +  if (s == NULL)
> +    {
> +      __pthread_force_elision = __elision_available;
> +      return;
> +    }
> +
> +  if (strncmp (s, "adaptive", 8) == 0 && (s[8] == 0 || s[8] == ':'))
> +    {
> +      __pthread_force_elision = __elision_available;
> +      if (s[8] == ':')
> +	elision_aconf_setup (s + 9);
> +    }
> +  else if (strncmp (s, "elision", 7) == 0 && (s[7] == 0 || s[7] == ':'))
> +    {
> +      __pthread_force_elision = __elision_available;
> +      if (s[7] == ':')
> +        elision_aconf_setup (s + 8);
> +    }

Why should we have two directives that do the same thing?  The tunables
aren't stable anyway, so there's no compatibility issue here.

Perhaps "adaptive" should be removed.  It's name isn't descriptive
anyway in combination with GLIBC_PTHREAD_MUTEX (ie, without _ELISION).

Note that depending on what we agree on regarding the semantics / stages
(see my reply to the whole patch set), this should be adapted.

> +  else if (strncmp (s, "none", 4) == 0 && s[4] == 0)

This should be "no_elision" or something like that.  A setting like
"GLIBC_PTHREAD_MUTEX=none" is confusing: no what? no mutexes?


> +    __pthread_force_elision = 0;
> +  else
> +    complain (PAIR ("pthreads: Unknown setting for GLIBC_PTHREAD_MUTEX\n"));
> +}
> +
> +/* Initialize elision rwlock.  */

Initialize lock elision for rwlocks.

> +
> +static void
> +elision_rwlock_init (const char *s)
> +{
> +  if (s == NULL)
> +    {
> +      __rwlock_rtm_enabled = __elision_available;
> +      return;
> +    }
> +  if (strncmp (s, "elision", 7) == 0)
> +    {
> +      __rwlock_rtm_enabled = __elision_available;
> +      if (s[7] == ':')
> +        {
> +          char *end;
> +	  int n;
> +
> +          n = strtoul (s + 8, &end, 0);
> +	  if (end == s + 8)
> +	    complain (PAIR ("pthreads: Bad retry number for GLIBC_PTHREAD_RWLOCK\n"));
> +          else
> +	    __rwlock_rtm_read_retries = n;
> +	}
> +    }
> +  else if (strcmp (s, "none") == 0)
> +    __rwlock_rtm_enabled = 0;
> +  else
> +    complain (PAIR ("pthreads: Unknown setting for GLIBC_PTHREAD_RWLOCK\n"));
> +}
> +
> +/* Initialize elison.  */
> +
> +static void
> +elision_init (int argc __attribute__ ((unused)),
> +	      char **argv  __attribute__ ((unused)),
> +	      char **environ)
> +{
> +  __elision_available = HAS_RTM;
> +
> +  /* For static builds need to call this explicitely. Noop for dynamic.  */
> +  __glibc_var_init (argc, argv, environ);
> +  elision_mutex_init (_dl_glibc_var[GLIBC_VAR_PTHREAD_MUTEX].val);
> +  elision_rwlock_init (_dl_glibc_var[GLIBC_VAR_PTHREAD_RWLOCK].val);
> +}
> +
> +#ifdef SHARED
> +# define INIT_SECTION ".init_array"
> +#else
> +# define INIT_SECTION ".preinit_array"
> +#endif
> +
> +void (*const __pthread_init_array []) (int, char **, char **)
> +  __attribute__ ((section (INIT_SECTION), aligned (sizeof (void *)))) =
> +{
> +  &elision_init
> +};
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.h b/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.h
> new file mode 100644
> index 0000000..9a540b9
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-conf.h
> @@ -0,0 +1,53 @@
> +/* elision-conf.h: Lock elision tunable parameters.
> +   Copyright (C) 2013 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C 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.
> +
> +   The GNU C 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 the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>. */
> +#ifndef _ELISION_CONF_H
> +#define _ELISION_CONF_H 1
> +
> +#include <pthread.h>
> +#include <cpuid.h>
> +#include <time.h>
> +
> +/* Should make sure there is no false sharing on this.  */
> +
> +struct elision_config
> +{
> +  int skip_lock_busy;
> +  int skip_lock_internal_abort;
> +  int retry_try_xbegin;
> +  int skip_trylock_internal_abort;
> +};
> +
> +extern struct elision_config __elision_aconf attribute_hidden;
> +
> +extern int __rwlock_rtm_enabled;
> +extern int __elision_available;
> +
> +extern int __pthread_mutex_timedlock_nortm (pthread_mutex_t *mutex, const struct timespec *);
> +extern int __pthread_mutex_timedlock_rtm (pthread_mutex_t *mutex, const struct timespec *);
> +extern int __pthread_mutex_timedlock (pthread_mutex_t *mutex, const struct timespec *);
> +extern int __pthread_mutex_lock_nortm (pthread_mutex_t *mutex);
> +extern int __pthread_mutex_lock_rtm (pthread_mutex_t *mutex);
> +extern int __pthread_mutex_lock (pthread_mutex_t *mutex);
> +extern int __pthread_mutex_trylock_nortm (pthread_mutex_t *);
> +extern int __pthread_mutex_trylock_rtm (pthread_mutex_t *);
> +extern int __pthread_mutex_trylock (pthread_mutex_t *);
> +
> +/* Tell the test suite to test elision for this architecture.  */
> +#define HAVE_ELISION 1
> +
> +#endif
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
> new file mode 100644
> index 0000000..faffe78
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
> @@ -0,0 +1,95 @@
> +/* elision-lock.c: Elided pthread mutex lock.
> +   Copyright (C) 2011, 2012, 2013 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C 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.
> +
> +   The GNU C 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 the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>. */
> +
> +#include <pthread.h>
> +#include "pthreadP.h"
> +#include "lowlevellock.h"
> +#include "hle.h"
> +#include <elision-conf.h>
> +
> +#if !defined(LLL_LOCK) && !defined(EXTRAARG)
> +/* Make sure the configuration code is always linked in for static
> +   libraries.  */
> +#include "elision-conf.c"
> +#endif
> +
> +#ifndef EXTRAARG
> +#define EXTRAARG
> +#endif
> +#ifndef LLL_LOCK
> +#define LLL_LOCK(a,b) lll_lock(a,b), 0
> +#endif
> +
> +#define aconf __elision_aconf
> +
> +/* Adaptive lock using transactions.
> +   By default the lock region is run as a transaction, and when it
> +   aborts or the lock is busy the lock adapts itself.  */
> +
> +int
> +__lll_lock_elision (int *futex, short *try_lock, EXTRAARG int private)
> +{
> +  if (*try_lock <= 0)
> +    {
> +      unsigned status;
> +      int try_xbegin;
> +
> +      for (try_xbegin = aconf.retry_try_xbegin;
> +	   try_xbegin > 0;
> +	   try_xbegin--)
> +	{
> +	  if ((status = _xbegin()) == _XBEGIN_STARTED)
> +	    {
> +	      if (*futex == 0)
> +		return 0;
> +
> +	      /* Lock was busy. Fall back to normal locking.
> +		 Could also _xend here but xabort with 0xff code
> +		 is more visible in the profiler.  */
> +	      _xabort (_ABORT_LOCK_BUSY);
> +	    }
> +
> +	  if (!(status & _XABORT_RETRY))
> +	    {
> +	      if ((status & _XABORT_EXPLICIT)
> +			&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
> +	        {
> +		  /* Right now we skip here. Better would be to wait a bit
> +		     and retry. This likely needs some spinning. */

whitespace

> +		  if (*try_lock != aconf.skip_lock_busy)
> +		    *try_lock = aconf.skip_lock_busy;
> +		}
> +	      /* Internal abort. There is no chance for retry.
> +		 Use the normal locking and next time use lock.
> +		 Be careful to avoid writing to the lock.  */
> +	      else if (*try_lock != aconf.skip_lock_internal_abort)
> +		*try_lock = aconf.skip_lock_internal_abort;
> +	      break;
> +	    }
> +	}
> +    }
> +  else
> +    {
> +      /* Use a normal lock until the threshold counter runs out.
> +	 Lost updates possible.  */
> +      (*try_lock)--;
> +    }
> +
> +  /* Use a normal lock here.  */
> +  return LLL_LOCK ((*futex), private);
> +}
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-timed.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-timed.c
> new file mode 100644
> index 0000000..3e95623
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-timed.c
> @@ -0,0 +1,8 @@

I suppose this needs a copyright header, if I remember past reviews
correctly.

> +#include <time.h>
> +#include <elision-conf.h>
> +#include "lowlevellock.h"
> +#define __lll_lock_elision __lll_timedlock_elision
> +#define EXTRAARG const struct timespec *t,
> +#undef LLL_LOCK
> +#define LLL_LOCK(a, b) lll_timedlock(a, t, b)
> +#include "elision-lock.c"
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
> new file mode 100644
> index 0000000..97b13b3
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
> @@ -0,0 +1,72 @@
> +/* elision-trylock.c: Lock eliding trylock for pthreads.
> +   Copyright (C) 2013 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C 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.
> +
> +   The GNU C 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 the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>. */
> +
> +#include <pthread.h>
> +#include <pthreadP.h>
> +#include <lowlevellock.h>
> +#include "hle.h"
> +#include <elision-conf.h>
> +
> +#define aconf __elision_aconf
> +
> +/* Try to elide a futex trylock. FUTEX is the futex variable. TRY_LOCK is the
> +   adaptation counter in the mutex. UPGRADED is != 0 when this is for an
> +   automatically upgraded lock.  */

You need to point to documentation about what "upgraded" is.  The
"upgrade" is rather about using by elision by default without changing
the mutex semantics, not an upgrade that would change semantics.  You
could also rename the parameter "posix_semantics" or something like
that.

> +
> +int
> +__lll_trylock_elision (int *futex, short *try_lock, int upgraded)
> +{
> +  /* Only try a transaction if it's worth it.  */
> +  if (*try_lock <= 0)
> +    {
> +      unsigned status;
> +
> +      /* When this could be a nested trylock that is not explicitely
> +	 declared an elided lock abort. This makes us follow POSIX
> +	 paper semantics.  */

Change the comment to:
If this could be a nested trylock and we need to follow current POSIX
semantics, then abort.

> +      if (upgraded)
> +        _xabort (_ABORT_NESTED_TRYLOCK);
> +
> +      if ((status = _xbegin()) == _XBEGIN_STARTED)
> +	{
> +	  if (*futex == 0)
> +	    return 0;
> +
> +	  /* Lock was busy. Fall back to normal locking.
> +	     Could also _xend here but xabort with 0xff code
> +	     is more visible in the profiler.  */
> +	  _xabort (_ABORT_LOCK_BUSY);
> +	}
> +
> +      if (!(status & _XABORT_RETRY))
> +        {
> +          /* Internal abort. No chance for retry. For future
> +             locks don't try speculation for some time.  */
> +          if (*try_lock != aconf.skip_trylock_internal_abort)
> +            *try_lock = aconf.skip_trylock_internal_abort;
> +        }
> +      /* Could do some retries here. */
> +    }
> +  else
> +    {
> +      /* Lost updates are possible, but harmless.  */
> +      (*try_lock)--;
> +    }
> +
> +  return lll_trylock (*futex);
> +}
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-unlock.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-unlock.c
> new file mode 100644
> index 0000000..bb13c6b
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-unlock.c
> @@ -0,0 +1,33 @@
> +/* elision-unlock.c: Commit an elided pthread lock.
> +   Copyright (C) 2013 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C 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.
> +
> +   The GNU C 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 the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#include "pthreadP.h"
> +#include "lowlevellock.h"
> +#include "hle.h"
> +
> +int
> +__lll_unlock_elision(int *lock, int private)
> +{
> +  /* When the lock was free we're in a transaction.
> +     When you crash here you unlocked a free lock.  */
> +  if (*lock == 0)
> +    _xend();
> +  else
> +    lll_unlock ((*lock), private);
> +  return 0;
> +}
> diff --git a/nptl/sysdeps/unix/sysv/linux/x86/hle.h b/nptl/sysdeps/unix/sysv/linux/x86/hle.h
> new file mode 100644
> index 0000000..a08f0fa
> --- /dev/null
> +++ b/nptl/sysdeps/unix/sysv/linux/x86/hle.h

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