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x86: Remove low-level lock optimization

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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.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Florian Weimer 2021-04-21 19:49:51 +02:00
parent 99f841c441
commit 60d5e40ab2
3 changed files with 0 additions and 89 deletions
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1
sysdeps/i386/nptl/pthread_spin_lock.S
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@ -16,7 +16,6 @@
<https://www.gnu.org/licenses/>. */
#include <sysdep.h>
#include <lowlevellock.h>
ENTRY (pthread_spin_lock)
mov 4(%esp), %eax

87
sysdeps/unix/sysv/linux/x86/lowlevellock.h
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@ -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 */

1
sysdeps/x86_64/nptl/pthread_spin_lock.S
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@ -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)