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a5544970 | 1 | /* Copyright (C) 2005-2019 Free Software Foundation, Inc. |
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2 | Contributed by Richard Henderson <rth@redhat.com>. |
3 | ||
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4 | This file is part of the GNU Offloading and Multi Processing Library |
5 | (libgomp). | |
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6 | |
7 | Libgomp is free software; you can redistribute it and/or modify it | |
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8 | under the terms of the GNU General Public License as published by |
9 | the Free Software Foundation; either version 3, or (at your option) | |
10 | any later version. | |
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11 | |
12 | Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY | |
13 | WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS | |
748086b7 | 14 | FOR A PARTICULAR PURPOSE. See the GNU General Public License for |
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15 | more details. |
16 | ||
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17 | Under Section 7 of GPL version 3, you are granted additional |
18 | permissions described in the GCC Runtime Library Exception, version | |
19 | 3.1, as published by the Free Software Foundation. | |
953ff289 | 20 | |
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21 | You should have received a copy of the GNU General Public License and |
22 | a copy of the GCC Runtime Library Exception along with this program; | |
23 | see the files COPYING3 and COPYING.RUNTIME respectively. If not, see | |
24 | <http://www.gnu.org/licenses/>. */ | |
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25 | |
26 | /* This is a Linux specific implementation of a semaphore synchronization | |
27 | mechanism for libgomp. This type is private to the library. This | |
28 | implementation uses atomic instructions and the futex syscall. */ | |
29 | ||
a68ab351 | 30 | #include "wait.h" |
953ff289 | 31 | |
953ff289 | 32 | void |
3e348fcc | 33 | gomp_sem_wait_slow (gomp_sem_t *sem, int count) |
953ff289 | 34 | { |
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35 | /* First loop spins a while. */ |
36 | while (count == 0) | |
37 | if (do_spin (sem, 0) | |
38 | /* Spin timeout, nothing changed. Set waiting flag. */ | |
39 | && __atomic_compare_exchange_n (sem, &count, SEM_WAIT, false, | |
40 | MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) | |
41 | { | |
42 | futex_wait (sem, SEM_WAIT); | |
43 | count = *sem; | |
44 | break; | |
45 | } | |
46 | /* Something changed. If it wasn't the wait flag, we're good to go. */ | |
47 | else if (__builtin_expect (((count = *sem) & SEM_WAIT) == 0 && count != 0, | |
48 | 1)) | |
49 | { | |
50 | if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, false, | |
51 | MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) | |
52 | return; | |
53 | } | |
54 | ||
55 | /* Second loop waits until semaphore is posted. We always exit this | |
56 | loop with wait flag set, so next post will awaken a thread. */ | |
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57 | while (1) |
58 | { | |
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59 | unsigned int wake = count & ~SEM_WAIT; |
60 | int newval = SEM_WAIT; | |
61 | ||
62 | if (wake != 0) | |
63 | newval |= wake - SEM_INC; | |
64 | if (__atomic_compare_exchange_n (sem, &count, newval, false, | |
65 | MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) | |
953ff289 | 66 | { |
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67 | if (wake != 0) |
68 | { | |
69 | /* If we can wake more threads, do so now. */ | |
70 | if (wake > SEM_INC) | |
71 | gomp_sem_post_slow (sem); | |
72 | break; | |
73 | } | |
74 | do_wait (sem, SEM_WAIT); | |
75 | count = *sem; | |
953ff289 | 76 | } |
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77 | } |
78 | } | |
79 | ||
80 | void | |
81 | gomp_sem_post_slow (gomp_sem_t *sem) | |
82 | { | |
3e348fcc | 83 | futex_wake (sem, 1); |
953ff289 | 84 | } |