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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
88 lines
2.8 KiB
C
88 lines
2.8 KiB
C
/* Low-level lock implementation, x86 version.
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Copyright (C) 2019-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef _X86_64_LOWLEVELLOCK_H
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#define _X86_64_LOWLEVELLOCK_H 1
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#ifndef __ASSEMBLER__
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#include <sysdeps/nptl/lowlevellock.h>
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#include <single-thread.h>
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/* The lll_trylock, lll_lock, and lll_unlock implements a single-thread
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optimization using the cmpxchgl instruction. It checks if the process
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is single thread and avoid a more expensive atomic instruction. */
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/* The single-thread optimization only works for libc itself, we need
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atomicity for libpthread in case of shared futexes. */
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#if !IS_IN(libc)
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# define is_single_thread 0
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#else
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# define is_single_thread SINGLE_THREAD_P
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#endif
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/* In the __lllc_as we simply return the value in %eax after the cmpxchg
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instruction. In case the operation succeeded this value is zero. In case
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the operation failed, the cmpxchg instruction has loaded the current value
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of the memory work which is guaranteed to be nonzero. */
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static inline int
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__attribute__ ((always_inline))
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__lll_cas_lock (int *futex)
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{
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int ret;
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asm volatile ("cmpxchgl %2, %1"
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: "=a" (ret), "=m" (*futex)
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: "r" (1), "m" (*futex), "0" (0)
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: "memory");
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return ret;
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}
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#undef lll_trylock
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#define lll_trylock(lock) \
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({ \
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int __ret; \
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if (is_single_thread) \
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__ret = __lll_cas_lock (&(lock)); \
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else \
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__ret = __lll_trylock (&(lock)); \
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__ret; \
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})
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#undef lll_lock
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#define lll_lock(lock, private) \
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((void) \
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({ \
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if (is_single_thread) \
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__lll_cas_lock (&(lock)); \
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else \
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__lll_lock (&(lock), private); \
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}))
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#undef lll_unlock
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#define lll_unlock(lock, private) \
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((void) \
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({ \
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if (is_single_thread) \
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(lock)--; \
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else \
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__lll_unlock (&(lock), private); \
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}))
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#endif /* !__ASSEMBLER__ */
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#endif /* lowlevellock.h */
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