1/* Low-level lock implementation, x86 version.
2 Copyright (C) 2019-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#ifndef _X86_64_LOWLEVELLOCK_H
20#define _X86_64_LOWLEVELLOCK_H 1
21
22#ifndef __ASSEMBLER__
23#include <sysdeps/nptl/lowlevellock.h>
24#include <single-thread.h>
25
26/* The lll_trylock, lll_lock, and lll_unlock implements a single-thread
27 optimization using the cmpxchgl instruction. It checks if the process
28 is single thread and avoid a more expensive atomic instruction. */
29
30/* The single-thread optimization only works for libc itself, we need
31 atomicity for libpthread in case of shared futexes. */
32#if !IS_IN(libc)
33# define is_single_thread 0
34#else
35# define is_single_thread SINGLE_THREAD_P
36#endif
37
38/* In the __lllc_as we simply return the value in %eax after the cmpxchg
39 instruction. In case the operation succeeded this value is zero. In case
40 the operation failed, the cmpxchg instruction has loaded the current value
41 of the memory work which is guaranteed to be nonzero. */
42static inline int
43__attribute__ ((always_inline))
44__lll_cas_lock (int *futex)
45{
46 int ret;
47 asm volatile ("cmpxchgl %2, %1"
48 : "=a" (ret), "=m" (*futex)
49 : "r" (1), "m" (*futex), "0" (0)
50 : "memory");
51 return ret;
52}
53
54#undef lll_trylock
55#define lll_trylock(lock) \
56 ({ \
57 int __ret; \
58 if (is_single_thread) \
59 __ret = __lll_cas_lock (&(lock)); \
60 else \
61 __ret = __lll_trylock (&(lock)); \
62 __ret; \
63 })
64
65#undef lll_lock
66#define lll_lock(lock, private) \
67 ((void) \
68 ({ \
69 if (is_single_thread) \
70 __lll_cas_lock (&(lock)); \
71 else \
72 __lll_lock (&(lock), private); \
73 }))
74
75#undef lll_unlock
76#define lll_unlock(lock, private) \
77 ((void) \
78 ({ \
79 if (is_single_thread) \
80 (lock)--; \
81 else \
82 __lll_unlock (&(lock), private); \
83 }))
84
85extern int __lll_clocklock_elision (int *futex, short *adapt_count,
86 clockid_t clockid,
87 const struct __timespec64 *timeout,
88 int private) attribute_hidden;
89
90#define lll_clocklock_elision(futex, adapt_count, clockid, timeout, private) \
91 __lll_clocklock_elision (&(futex), &(adapt_count), clockid, timeout, private)
92
93extern int __lll_lock_elision (int *futex, short *adapt_count, int private)
94 attribute_hidden;
95
96extern int __lll_unlock_elision (int *lock, int private)
97 attribute_hidden;
98
99extern int __lll_trylock_elision (int *lock, short *adapt_count)
100 attribute_hidden;
101
102#define lll_lock_elision(futex, adapt_count, private) \
103 __lll_lock_elision (&(futex), &(adapt_count), private)
104#define lll_unlock_elision(futex, adapt_count, private) \
105 __lll_unlock_elision (&(futex), private)
106#define lll_trylock_elision(futex, adapt_count) \
107 __lll_trylock_elision (&(futex), &(adapt_count))
108
109#endif /* !__ASSEMBLER__ */
110
111#endif /* lowlevellock.h */
112