[v3,33/37] x86: Remove low-level lock optimization

  The current approach is to do this optimizations at a higher level,
in generic code, so that single-threaded cases can be specifically
targeted.

Furthermore, using IS_IN (libc) as a compile-time indicator that
all locks are private is no longer correct once process-shared lock
implementations are moved into libc.

The generic <lowlevellock.h> is not compatible with assembler code
(obviously), so it's necessary to remove two long-unused #includes.
---
 sysdeps/i386/nptl/pthread_spin_lock.S      |  1 -
 sysdeps/unix/sysv/linux/x86/lowlevellock.h | 87 ----------------------
 sysdeps/x86_64/nptl/pthread_spin_lock.S    |  1 -
 3 files changed, 89 deletions(-)
 delete mode 100644 sysdeps/unix/sysv/linux/x86/lowlevellock.h
  

On 16/03/2021 14:31, Florian Weimer via Libc-alpha wrote:
> The current approach is to do this optimizations at a higher level,
> in generic code, so that single-threaded cases can be specifically
> targeted.
> 
> Furthermore, using IS_IN (libc) as a compile-time indicator that
> all locks are private is no longer correct once process-shared lock
> implementations are moved into libc.
> 
> The generic <lowlevellock.h> is not compatible with assembler code
> (obviously), so it's necessary to remove two long-unused #includes.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  sysdeps/i386/nptl/pthread_spin_lock.S      |  1 -
>  sysdeps/unix/sysv/linux/x86/lowlevellock.h | 87 ----------------------
>  sysdeps/x86_64/nptl/pthread_spin_lock.S    |  1 -
>  3 files changed, 89 deletions(-)
>  delete mode 100644 sysdeps/unix/sysv/linux/x86/lowlevellock.h
> 
> diff --git a/sysdeps/i386/nptl/pthread_spin_lock.S b/sysdeps/i386/nptl/pthread_spin_lock.S
> index eec157fde7..985b236aae 100644
> --- a/sysdeps/i386/nptl/pthread_spin_lock.S
> +++ b/sysdeps/i386/nptl/pthread_spin_lock.S
> @@ -16,7 +16,6 @@
>     <https://www.gnu.org/licenses/>.  */
>  
>  #include <sysdep.h>
> -#include <lowlevellock.h>
>  
>  ENTRY (pthread_spin_lock)
>  	mov	4(%esp), %eax

Ok.

> diff --git a/sysdeps/unix/sysv/linux/x86/lowlevellock.h b/sysdeps/unix/sysv/linux/x86/lowlevellock.h
> deleted file mode 100644
> index 7205bcadfd..0000000000
> --- a/sysdeps/unix/sysv/linux/x86/lowlevellock.h
> +++ /dev/null
> @@ -1,87 +0,0 @@
> -/* Low-level lock implementation, x86 version.
> -   Copyright (C) 2019-2021 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
> -   <https://www.gnu.org/licenses/>.  */
> -
> -#ifndef _X86_64_LOWLEVELLOCK_H
> -#define _X86_64_LOWLEVELLOCK_H	1
> -
> -#ifndef __ASSEMBLER__
> -#include <sysdeps/nptl/lowlevellock.h>
> -#include <single-thread.h>
> -
> -/* The lll_trylock, lll_lock, and lll_unlock implements a single-thread
> -   optimization using the cmpxchgl instruction.  It checks if the process
> -   is single thread and avoid a more expensive atomic instruction.  */
> -
> -/* The single-thread optimization only works for libc itself, we need
> -   atomicity for libpthread in case of shared futexes.  */
> -#if !IS_IN(libc)
> -# define is_single_thread 0
> -#else
> -# define is_single_thread SINGLE_THREAD_P
> -#endif
> -
> -/* In the __lllc_as we simply return the value in %eax after the cmpxchg
> -   instruction.  In case the operation succeeded this value is zero.  In case
> -   the operation failed, the cmpxchg instruction has loaded the current value
> -   of the memory work which is guaranteed to be nonzero.  */
> -static inline int
> -__attribute__ ((always_inline))
> -__lll_cas_lock (int *futex)
> -{
> -  int ret;
> -  asm volatile ("cmpxchgl %2, %1"
> -		: "=a" (ret), "=m" (*futex)
> -		: "r" (1), "m" (*futex), "0" (0)
> -		: "memory");
> -  return ret;
> -}
> -
> -#undef lll_trylock
> -#define lll_trylock(lock)						     \
> -  ({									     \
> -    int __ret;								     \
> -    if (is_single_thread)						     \
> -      __ret = __lll_cas_lock (&(lock));					     \
> -    else								     \
> -      __ret = __lll_trylock (&(lock));					     \
> -    __ret;								     \
> -  })
> -
> -#undef lll_lock
> -#define lll_lock(lock, private)						     \
> -  ((void)								     \
> -   ({									     \
> -     if (is_single_thread)						     \
> -       __lll_cas_lock (&(lock));					     \
> -     else								     \
> -       __lll_lock (&(lock), private);					     \
> -   }))
> -
> -#undef lll_unlock
> -#define lll_unlock(lock, private)					     \
> -  ((void)								     \
> -   ({									     \
> -     if (is_single_thread)						     \
> -       (lock)--;							     \
> -     else								     \
> -       __lll_unlock (&(lock), private);					     \
> -   }))
> -
> -#endif  /* !__ASSEMBLER__ */
> -
> -#endif	/* lowlevellock.h */

Ok.

> diff --git a/sysdeps/x86_64/nptl/pthread_spin_lock.S b/sysdeps/x86_64/nptl/pthread_spin_lock.S
> index 66cbbc5252..ff5e7525ca 100644
> --- a/sysdeps/x86_64/nptl/pthread_spin_lock.S
> +++ b/sysdeps/x86_64/nptl/pthread_spin_lock.S
> @@ -15,7 +15,6 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> -#include <lowlevellock.h>
>  #include <sysdep.h>
>  
>  ENTRY(pthread_spin_lock)
> 

Ok.