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04277e02 | 1 | /* Copyright (C) 2005-2019 Free Software Foundation, Inc. |
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2 | This file is part of the GNU C Library. |
3 | ||
4 | The GNU C Library is free software; you can redistribute it and/or | |
5 | modify it under the terms of the GNU Lesser General Public | |
6 | License as published by the Free Software Foundation; either | |
7 | version 2.1 of the License, or (at your option) any later version. | |
8 | ||
9 | The GNU C Library is distributed in the hope that it will be useful, | |
10 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
11 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
12 | Lesser General Public License for more details. | |
13 | ||
14 | You should have received a copy of the GNU Lesser General Public | |
ab84e3ff | 15 | License along with the GNU C Library. If not, see |
5a82c748 | 16 | <https://www.gnu.org/licenses/>. */ |
da1ea0f0 | 17 | |
da1ea0f0 | 18 | #include "pthreadP.h" |
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19 | |
20 | int | |
21 | pthread_spin_unlock (pthread_spinlock_t *lock) | |
22 | { | |
d7f91484 JDA |
23 | /* CONCURRENCTY NOTES: |
24 | ||
25 | The atomic_exchange_rel synchronizes-with the atomic_exhange_acq in | |
26 | pthread_spin_lock. | |
27 | ||
28 | On hppa we must not use a plain `stw` to reset the guard lock. This | |
29 | has to do with the kernel compare-and-swap helper that is used to | |
30 | implement all of the atomic operations. | |
31 | ||
32 | The kernel CAS helper uses its own internal locks and that means that | |
33 | to create a true happens-before relationship between any two threads, | |
34 | the second thread must observe the internal lock having a value of 0 | |
35 | (it must attempt to take the lock with ldcw). This creates the | |
36 | ordering required for a second thread to observe the effects of the | |
37 | RMW of the kernel CAS helper in any other thread. | |
38 | ||
39 | Therefore if a variable is used in an atomic macro it must always be | |
40 | manipulated with atomic macros in order for memory ordering rules to | |
41 | be preserved. */ | |
42 | atomic_exchange_rel (lock, 0); | |
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43 | return 0; |
44 | } |