1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
|
/******************************************************
Mutex, the basic synchronization primitive
(c) 1995 Innobase Oy
Created 9/5/1995 Heikki Tuuri
*******************************************************/
/**********************************************************************
Sets the waiters field in a mutex. */
void
mutex_set_waiters(
/*==============*/
mutex_t* mutex, /* in: mutex */
ulint n); /* in: value to set */
/**********************************************************************
Reserves a mutex for the current thread. If the mutex is reserved, the
function spins a preset time (controlled by SYNC_SPIN_ROUNDS) waiting
for the mutex before suspending the thread. */
void
mutex_spin_wait(
/*============*/
mutex_t* mutex, /* in: pointer to mutex */
char* file_name,/* in: file name where mutex requested */
ulint line); /* in: line where requested */
#ifdef UNIV_SYNC_DEBUG
/**********************************************************************
Sets the debug information for a reserved mutex. */
void
mutex_set_debug_info(
/*=================*/
mutex_t* mutex, /* in: mutex */
char* file_name, /* in: file where requested */
ulint line); /* in: line where requested */
#endif /* UNIV_SYNC_DEBUG */
/**********************************************************************
Releases the threads waiting in the primary wait array for this mutex. */
void
mutex_signal_object(
/*================*/
mutex_t* mutex); /* in: mutex */
/**********************************************************************
Performs an atomic test-and-set instruction to the lock_word field of a
mutex. */
UNIV_INLINE
ulint
mutex_test_and_set(
/*===============*/
/* out: the previous value of lock_word: 0 or
1 */
mutex_t* mutex) /* in: mutex */
{
#if defined(_WIN32) && defined(UNIV_CAN_USE_X86_ASSEMBLER)
ulint res;
ulint* lw; /* assembler code is used to ensure that
lock_word is loaded from memory */
ut_ad(mutex);
ut_ad(sizeof(ulint) == 4);
lw = &(mutex->lock_word);
__asm MOV ECX, lw
__asm MOV EDX, 1
__asm XCHG EDX, DWORD PTR [ECX]
__asm MOV res, EDX
/* The fence below would prevent this thread from reading the data
structure protected by the mutex before the test-and-set operation is
committed, but the fence is apparently not needed:
In a posting to comp.arch newsgroup (August 10, 1997) Andy Glew said
that in P6 a LOCKed instruction like XCHG establishes a fence with
respect to memory reads and writes and thus an explicit fence is not
needed. In P5 he seemed to agree with a previous newsgroup poster that
LOCKed instructions serialize all instruction execution, and,
consequently, also memory operations. This is confirmed in Intel
Software Dev. Manual, Vol. 3. */
/* mutex_fence(); */
return(res);
#elif defined(not_defined) && defined(__GNUC__) && defined(UNIV_INTEL_X86)
ulint* lw;
ulint res;
lw = &(mutex->lock_word);
/* In assembly we use the so-called AT & T syntax where
the order of operands is inverted compared to the ordinary Intel
syntax. The 'l' after the mnemonics denotes a 32-bit operation.
The line after the code tells which values come out of the asm
code, and the second line tells the input to the asm code. */
asm volatile("movl $1, %%eax; xchgl (%%ecx), %%eax" :
"=eax" (res), "=m" (*lw) :
"ecx" (lw));
return(res);
#else
ibool ret;
ret = os_fast_mutex_trylock(&(mutex->os_fast_mutex));
if (ret == 0) {
/* We check that os_fast_mutex_trylock does not leak
and allow race conditions */
ut_a(mutex->lock_word == 0);
mutex->lock_word = 1;
}
return(ret);
#endif
}
/**********************************************************************
Performs a reset instruction to the lock_word field of a mutex. This
instruction also serializes memory operations to the program order. */
UNIV_INLINE
void
mutex_reset_lock_word(
/*==================*/
mutex_t* mutex) /* in: mutex */
{
#if defined(_WIN32) && defined(UNIV_CAN_USE_X86_ASSEMBLER)
ulint* lw; /* assembler code is used to ensure that
lock_word is loaded from memory */
ut_ad(mutex);
lw = &(mutex->lock_word);
__asm MOV EDX, 0
__asm MOV ECX, lw
__asm XCHG EDX, DWORD PTR [ECX]
#elif defined(not_defined) && defined(__GNUC__) && defined(UNIV_INTEL_X86)
ulint* lw;
lw = &(mutex->lock_word);
/* In assembly we use the so-called AT & T syntax where
the order of operands is inverted compared to the ordinary Intel
syntax. The 'l' after the mnemonics denotes a 32-bit operation. */
asm volatile("movl $0, %%eax; xchgl (%%ecx), %%eax" :
"=m" (*lw) :
"ecx" (lw) :
"eax"); /* gcc does not seem to understand
that our asm code resets eax: tell it
explicitly that after the third ':' */
#else
mutex->lock_word = 0;
os_fast_mutex_unlock(&(mutex->os_fast_mutex));
#endif
}
/**********************************************************************
Gets the value of the lock word. */
UNIV_INLINE
ulint
mutex_get_lock_word(
/*================*/
mutex_t* mutex) /* in: mutex */
{
volatile ulint* ptr; /* declared volatile to ensure that
lock_word is loaded from memory */
ut_ad(mutex);
ptr = &(mutex->lock_word);
return(*ptr);
}
/**********************************************************************
Gets the waiters field in a mutex. */
UNIV_INLINE
ulint
mutex_get_waiters(
/*==============*/
/* out: value to set */
mutex_t* mutex) /* in: mutex */
{
volatile ulint* ptr; /* declared volatile to ensure that
the value is read from memory */
ut_ad(mutex);
ptr = &(mutex->waiters);
return(*ptr); /* Here we assume that the read of a single
word from memory is atomic */
}
/**********************************************************************
Unlocks a mutex owned by the current thread. */
UNIV_INLINE
void
mutex_exit(
/*=======*/
mutex_t* mutex) /* in: pointer to mutex */
{
#ifdef UNIV_SYNC_DEBUG
ut_ad(mutex_own(mutex));
mutex->thread_id = ULINT_UNDEFINED;
sync_thread_reset_level(mutex);
#endif
mutex_reset_lock_word(mutex);
/* A problem: we assume that mutex_reset_lock word
is a memory barrier, that is when we read the waiters
field next, the read must be serialized in memory
after the reset. A speculative processor might
perform the read first, which could leave a waiting
thread hanging indefinitely.
Our current solution call every 10 seconds
sync_arr_wake_threads_if_sema_free()
to wake up possible hanging threads if
they are missed in mutex_signal_object. */
if (mutex_get_waiters(mutex) != 0) {
mutex_signal_object(mutex);
}
#ifdef UNIV_SYNC_PERF_STAT
mutex_exit_count++;
#endif
}
/**********************************************************************
Locks a mutex for the current thread. If the mutex is reserved, the function
spins a preset time (controlled by SYNC_SPIN_ROUNDS), waiting for the mutex
before suspending the thread. */
UNIV_INLINE
void
mutex_enter_func(
/*=============*/
mutex_t* mutex, /* in: pointer to mutex */
char* file_name,/* in: file name where locked */
ulint line) /* in: line where locked */
{
ut_ad(mutex_validate(mutex));
/* Note that we do not peek at the value of lock_word before trying
the atomic test_and_set; we could peek, and possibly save time. */
if (!mutex_test_and_set(mutex)) {
#ifdef UNIV_SYNC_DEBUG
mutex_set_debug_info(mutex, file_name, line);
#endif
return; /* Succeeded! */
}
mutex_spin_wait(mutex, file_name, line);
}
|
