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1 | /* |
1 | /* |
2 | * Copyright (c) 2008 Jakub Jermar |
2 | * Copyright (c) 2008 Jakub Jermar |
3 | * All rights reserved. |
3 | * All rights reserved. |
4 | * |
4 | * |
5 | * Redistribution and use in source and binary forms, with or without |
5 | * Redistribution and use in source and binary forms, with or without |
6 | * modification, are permitted provided that the following conditions |
6 | * modification, are permitted provided that the following conditions |
7 | * are met: |
7 | * are met: |
8 | * |
8 | * |
9 | * - Redistributions of source code must retain the above copyright |
9 | * - Redistributions of source code must retain the above copyright |
10 | * notice, this list of conditions and the following disclaimer. |
10 | * notice, this list of conditions and the following disclaimer. |
11 | * - Redistributions in binary form must reproduce the above copyright |
11 | * - Redistributions in binary form must reproduce the above copyright |
12 | * notice, this list of conditions and the following disclaimer in the |
12 | * notice, this list of conditions and the following disclaimer in the |
13 | * documentation and/or other materials provided with the distribution. |
13 | * documentation and/or other materials provided with the distribution. |
14 | * - The name of the author may not be used to endorse or promote products |
14 | * - The name of the author may not be used to endorse or promote products |
15 | * derived from this software without specific prior written permission. |
15 | * derived from this software without specific prior written permission. |
16 | * |
16 | * |
17 | * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
17 | * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
18 | * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
18 | * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
19 | * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
19 | * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
20 | * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
20 | * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
21 | * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
21 | * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
22 | * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
22 | * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
23 | * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
23 | * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
24 | * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
24 | * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
25 | * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
25 | * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
26 | * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
26 | * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
27 | */ |
27 | */ |
28 | 28 | ||
29 | /** @addtogroup libc |
29 | /** @addtogroup libc |
30 | * @{ |
30 | * @{ |
31 | */ |
31 | */ |
32 | /** @file |
32 | /** @file |
33 | */ |
33 | */ |
34 | 34 | ||
35 | #include <futex.h> |
35 | #include <futex.h> |
36 | #include <atomic.h> |
36 | #include <atomic.h> |
37 | #include <libc.h> |
37 | #include <libc.h> |
38 | #include <stdio.h> |
38 | #include <stdio.h> |
39 | #include <sys/types.h> |
39 | #include <sys/types.h> |
40 | #include <kernel/synch/synch.h> |
40 | #include <kernel/synch/synch.h> |
41 | 41 | ||
42 | /* |
42 | /* |
43 | * Note about race conditions. |
43 | * Note about race conditions. |
44 | * Because of non-atomic nature of operations performed sequentially on the |
44 | * Because of non-atomic nature of operations performed sequentially on the |
45 | * futex counter and the futex wait queue, there is a race condition: |
45 | * futex counter and the futex wait queue, there is a race condition: |
46 | * |
46 | * |
47 | * (wq->missed_wakeups == 1) && (futex->count = 1) |
47 | * (wq->missed_wakeups == 1) && (futex->count = 1) |
48 | * |
48 | * |
49 | * Scenario 1 (wait queue timeout vs. futex_up()): |
49 | * Scenario 1 (wait queue timeout vs. futex_up()): |
50 | * 1. assume wq->missed_wakeups == 0 && futex->count == -1 |
50 | * 1. assume wq->missed_wakeups == 0 && futex->count == -1 |
51 | * (ie. thread A sleeping, thread B in the critical section) |
51 | * (ie. thread A sleeping, thread B in the critical section) |
52 | * 2. A receives timeout and gets removed from the wait queue |
52 | * 2. A receives timeout and gets removed from the wait queue |
53 | * 3. B wants to leave the critical section and calls futex_up() |
53 | * 3. B wants to leave the critical section and calls futex_up() |
54 | * 4. B thus changes futex->count from -1 to 0 |
54 | * 4. B thus changes futex->count from -1 to 0 |
55 | * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread |
55 | * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread |
56 | * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1 |
56 | * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1 |
57 | * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it |
57 | * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it |
58 | * from 0 to 1 |
58 | * from 0 to 1 |
59 | * |
59 | * |
60 | * Scenario 2 (conditional down operation vs. futex_up) |
60 | * Scenario 2 (conditional down operation vs. futex_up) |
61 | * 1. assume wq->missed_wakeups == 0 && futex->count == 0 |
61 | * 1. assume wq->missed_wakeups == 0 && futex->count == 0 |
62 | * (i.e. thread A is in the critical section) |
62 | * (i.e. thread A is in the critical section) |
63 | * 2. thread B performs futex_trydown() operation and changes futex->count from |
63 | * 2. thread B performs futex_trydown() operation and changes futex->count from |
64 | * 0 to -1 |
64 | * 0 to -1 |
65 | * B is now obliged to call SYS_FUTEX_SLEEP syscall |
65 | * B is now obliged to call SYS_FUTEX_SLEEP syscall |
66 | * 3. A wants to leave the critical section and does futex_up() |
66 | * 3. A wants to leave the critical section and does futex_up() |
67 | * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP |
67 | * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP |
68 | * syscall |
68 | * syscall |
69 | * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep |
69 | * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep |
70 | * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from |
70 | * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from |
71 | * 0 to 1 |
71 | * 0 to 1 |
72 | * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it |
72 | * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it |
73 | * from 0 to 1 |
73 | * from 0 to 1 |
74 | * |
74 | * |
75 | * Both scenarios allow two threads to be in the critical section |
75 | * Both scenarios allow two threads to be in the critical section |
76 | * simultaneously. One without kernel intervention and the other through |
76 | * simultaneously. One without kernel intervention and the other through |
77 | * wq->missed_wakeups being 1. |
77 | * wq->missed_wakeups being 1. |
78 | * |
78 | * |
79 | * To mitigate this problem, futex_down_timeout() detects that the syscall |
79 | * To mitigate this problem, futex_down_timeout() detects that the syscall |
80 | * didn't sleep in the wait queue, fixes the futex counter and RETRIES the |
80 | * didn't sleep in the wait queue, fixes the futex counter and RETRIES the |
81 | * whole operation again. |
81 | * whole operation again. |
82 | */ |
82 | */ |
83 | 83 | ||
84 | /** Initialize futex counter. |
84 | /** Initialize futex counter. |
85 | * |
85 | * |
86 | * @param futex Futex. |
86 | * @param futex Futex. |
87 | * @param val Initialization value. |
87 | * @param val Initialization value. |
88 | */ |
88 | */ |
89 | void futex_initialize(futex_t *futex, int val) |
89 | void futex_initialize(futex_t *futex, int val) |
90 | { |
90 | { |
91 | atomic_set(futex, val); |
91 | atomic_set(futex, val); |
92 | } |
92 | } |
93 | 93 | ||
94 | int futex_down(futex_t *futex) |
94 | int futex_down(futex_t *futex) |
95 | { |
95 | { |
96 | return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE); |
96 | return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE); |
97 | } |
97 | } |
98 | 98 | ||
99 | int futex_trydown(futex_t *futex) |
99 | int futex_trydown(futex_t *futex) |
100 | { |
100 | { |
101 | return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, |
101 | return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, |
102 | SYNCH_FLAGS_NON_BLOCKING); |
102 | SYNCH_FLAGS_NON_BLOCKING); |
103 | } |
103 | } |
104 | 104 | ||
105 | /** Try to down the futex. |
105 | /** Try to down the futex. |
106 | * |
106 | * |
107 | * @param futex Futex. |
107 | * @param futex Futex. |
108 | * @param usec Microseconds to wait. Zero value means sleep without |
108 | * @param usec Microseconds to wait. Zero value means sleep without |
109 | * timeout. |
109 | * timeout. |
110 | * @param flags Select mode of operation. See comment for |
110 | * @param flags Select mode of operation. See comment for |
111 | * waitq_sleep_timeout(). |
111 | * waitq_sleep_timeout(). |
112 | * |
112 | * |
113 | * @return ENOENT if there is no such virtual address. One of |
113 | * @return ENOENT if there is no such virtual address. One of |
114 | * ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or |
114 | * ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or |
115 | * ESYNCH_TIMEOUT if the lock was not acquired because of |
115 | * ESYNCH_TIMEOUT if the lock was not acquired because of |
116 | * a timeout or ESYNCH_WOULD_BLOCK if the operation could |
116 | * a timeout or ESYNCH_WOULD_BLOCK if the operation could |
117 | * not be carried out atomically (if requested so). |
117 | * not be carried out atomically (if requested so). |
118 | */ |
118 | */ |
119 | int futex_down_timeout(futex_t *futex, uint32_t usec, int flags) |
119 | int futex_down_timeout(futex_t *futex, uint32_t usec, int flags) |
120 | { |
120 | { |
121 | int rc; |
121 | int rc; |
122 | 122 | ||
123 | while (atomic_predec(futex) < 0) { |
123 | while (atomic_predec(futex) < 0) { |
124 | rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, |
124 | rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, |
125 | (sysarg_t) usec, (sysarg_t) flags); |
125 | (sysarg_t) usec, (sysarg_t) flags); |
126 | 126 | ||
127 | switch (rc) { |
127 | switch (rc) { |
128 | case ESYNCH_OK_ATOMIC: |
128 | case ESYNCH_OK_ATOMIC: |
129 | /* |
129 | /* |
130 | * Because of a race condition between timeout and |
130 | * Because of a race condition between timeout and |
131 | * futex_up() and between conditional |
131 | * futex_up() and between conditional |
132 | * futex_down_timeout() and futex_up(), we have to give |
132 | * futex_down_timeout() and futex_up(), we have to give |
133 | * up and try again in this special case. |
133 | * up and try again in this special case. |
134 | */ |
134 | */ |
135 | atomic_inc(futex); |
135 | atomic_inc(futex); |
136 | break; |
136 | break; |
137 | 137 | ||
138 | case ESYNCH_TIMEOUT: |
138 | case ESYNCH_TIMEOUT: |
139 | atomic_inc(futex); |
139 | atomic_inc(futex); |
140 | return ESYNCH_TIMEOUT; |
140 | return ESYNCH_TIMEOUT; |
141 | break; |
141 | break; |
142 | 142 | ||
143 | case ESYNCH_WOULD_BLOCK: |
143 | case ESYNCH_WOULD_BLOCK: |
144 | /* |
144 | /* |
145 | * The conditional down operation should be implemented |
145 | * The conditional down operation should be implemented |
146 | * this way. The userspace-only variant tends to |
146 | * this way. The userspace-only variant tends to |
147 | * accumulate missed wakeups in the kernel futex wait |
147 | * accumulate missed wakeups in the kernel futex wait |
148 | * queue. |
148 | * queue. |
149 | */ |
149 | */ |
150 | atomic_inc(futex); |
150 | atomic_inc(futex); |
151 | return ESYNCH_WOULD_BLOCK; |
151 | return ESYNCH_WOULD_BLOCK; |
152 | break; |
152 | break; |
153 | 153 | ||
154 | case ESYNCH_OK_BLOCKED: |
154 | case ESYNCH_OK_BLOCKED: |
155 | /* |
155 | /* |
156 | * Enter the critical section. |
156 | * Enter the critical section. |
157 | * The futex counter has already been incremented for |
157 | * The futex counter has already been incremented for |
158 | * us. |
158 | * us. |
159 | */ |
159 | */ |
160 | return ESYNCH_OK_BLOCKED; |
160 | return ESYNCH_OK_BLOCKED; |
161 | break; |
161 | break; |
162 | default: |
162 | default: |
163 | return rc; |
163 | return rc; |
164 | } |
164 | } |
165 | } |
165 | } |
166 | 166 | ||
167 | /* |
167 | /* |
168 | * Enter the critical section. |
168 | * Enter the critical section. |
169 | */ |
169 | */ |
170 | return ESYNCH_OK_ATOMIC; |
170 | return ESYNCH_OK_ATOMIC; |
171 | } |
171 | } |
172 | 172 | ||
173 | /** Up the futex. |
173 | /** Up the futex. |
174 | * |
174 | * |
175 | * @param futex Futex. |
175 | * @param futex Futex. |
176 | * |
176 | * |
177 | * @return ENOENT if there is no such virtual address. Otherwise |
177 | * @return ENOENT if there is no such virtual address. Otherwise |
178 | * zero. |
178 | * zero. |
179 | */ |
179 | */ |
180 | int futex_up(futex_t *futex) |
180 | int futex_up(futex_t *futex) |
181 | { |
181 | { |
182 | long val; |
182 | long val; |
183 | 183 | ||
184 | val = atomic_postinc(futex); |
184 | val = atomic_postinc(futex); |
185 | if (val < 0) |
185 | if (val < 0) |
186 | return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count); |
186 | return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count); |
187 | 187 | ||
188 | return 0; |
188 | return 0; |
189 | } |
189 | } |
190 | 190 | ||
191 | /** @} |
191 | /** @} |
192 | */ |
192 | */ |
193 | 193 |