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
|
/*
* The contents of this file are subject to the AOLserver Public License
* Version 1.1 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
* http://aolserver.com/.
*
* Software distributed under the License is distributed on an "AS IS"
* basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
* the License for the specific language governing rights and limitations
* under the License.
*
* The Original Code is AOLserver Code and related documentation
* distributed by AOL.
*
* The Initial Developer of the Original Code is America Online,
* Inc. Portions created by AOL are Copyright (C) 1999 America Online,
* Inc. All Rights Reserved.
*
* Alternatively, the contents of this file may be used under the terms
* of the GNU General Public License (the "GPL"), in which case the
* provisions of GPL are applicable instead of those above. If you wish
* to allow use of your version of this file only under the terms of the
* GPL and not to allow others to use your version of this file under the
* License, indicate your decision by deleting the provisions above and
* replace them with the notice and other provisions required by the GPL.
* If you do not delete the provisions above, a recipient may use your
* version of this file under either the License or the GPL.
*/
/*
* cs.c --
*
* Support for nsthread critical sections. Critical sections differ
* from mutex objects in that a critical section can be repeatedly
* locked by the same thread as long as each lock is matched with
* a cooresponding unlock. Critical sections are used cases where
* the lock could be called recursively.
*
* Note: Critical sections are almost always a bad idea. You'll
* see below that the number of actual lock and unlock operations are
* doubled and threads can end up in condition waits instead of spin
* locks. As a rule, if you think you need a critical section you
* probably haven't solved your problem correctly.
*/
static const char *RCSID = "@(#) $Header: /cvsroot/aolserver/aolserver/thread/cs.c,v 1.4 2000/11/06 17:53:50 jgdavidson Exp $, compiled: " __DATE__ " " __TIME__;
#include "thread.h"
/*
* The following structure defines a critcal section object which includes
* a lock, a pointer to the current thread which owns the critical section,
* and a condiation variable for waiting threads.
*/
typedef struct {
Ns_Mutex lock;
Ns_Cond cond;
Thread *ownerPtr;
int count;
} Cs;
/*
*----------------------------------------------------------------------
*
* Ns_CsInit --
*
* Initialize a critical section object.
*
* Results:
* None.
*
* Side effects:
* A critical section object is allocated from the heap.
*
*----------------------------------------------------------------------
*/
void
Ns_CsInit(Ns_Cs *csPtr)
{
Cs *cPtr;
cPtr = NsAlloc(sizeof(Cs));
Ns_MutexInit2(&cPtr->lock, "nsthread:cs");
Ns_CondInit(&cPtr->cond);
cPtr->count = 0;
cPtr->ownerPtr = NULL;
*csPtr = (Ns_Cs) cPtr;
}
/*
*----------------------------------------------------------------------
*
* Ns_CsDestroy --
*
* Destroy a critical section object. Note that you would almost
* never need to call this function as synchronization objects are
* typically created at startup and exist until the server exists.
*
* Results:
* None.
*
* Side effects:
* The underly objects in the critical section are destroy and
* the critical section memory returned to the heap.
*
*----------------------------------------------------------------------
*/
void
Ns_CsDestroy(Ns_Cs *csPtr)
{
/*
* Destroy the condition only if it's not null, i.e., initialized
* by the first use.
*/
if (*csPtr != NULL) {
Cs *cPtr = (Cs *) *csPtr;
Ns_MutexDestroy(&cPtr->lock);
Ns_CondDestroy(&cPtr->cond);
cPtr->ownerPtr = NULL;
cPtr->count = 0;
NsFree(cPtr);
*csPtr = NULL;
}
}
/*
*----------------------------------------------------------------------
*
* Ns_CsEnter --
*
* Lock a critical section object, initializing it first if needed.
*
* Results:
* None.
*
* Side effects:
* Thread may wait on the critical section condition variable if
* the critical section is already owned by another thread.
*
*----------------------------------------------------------------------
*/
void
Ns_CsEnter(Ns_Cs *csPtr)
{
Cs *cPtr;
Thread *thisPtr = NsGetThread();
/*
* Initialize the critical section if it has never been used before.
*/
if (*csPtr == NULL) {
Ns_MasterLock();
if (*csPtr == NULL) {
Ns_CsInit(csPtr);
}
Ns_MasterUnlock();
}
cPtr = (Cs *) *csPtr;
Ns_MutexLock(&cPtr->lock);
/*
* Wait on the condition if the critical section is owned by another
* thread.
*/
while (cPtr->count > 0 && cPtr->ownerPtr != thisPtr) {
Ns_CondWait(&cPtr->cond, &cPtr->lock);
}
cPtr->ownerPtr = thisPtr;
cPtr->count++;
Ns_MutexUnlock(&cPtr->lock);
}
/*
*----------------------------------------------------------------------
*
* Ns_CsLeave --
*
* Unlock a critical section once.
*
* Results:
* None.
*
* Side effects:
* Condition is signaled if this is the final unlock of the critical
* section.
*
*----------------------------------------------------------------------
*/
void
Ns_CsLeave(Ns_Cs *csPtr)
{
Cs *cPtr = (Cs *) *csPtr;
Thread *thisPtr = NsGetThread();
Ns_MutexLock(&cPtr->lock);
if (cPtr->ownerPtr == thisPtr && --cPtr->count == 0) {
cPtr->ownerPtr = NULL;
Ns_CondSignal(&cPtr->cond);
}
Ns_MutexUnlock(&cPtr->lock);
}
|
