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Make shared control block agnostic to 32-bit/64-bit.
We need to support cross-architecture host/child process. Use Futex on Linux which is a fixed 32-bit lock mechanism, unlike pthreads.
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| Original file line number | Diff line number | Diff line change |
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| /* Copyright (c) 2019 Hans-Kristian Arntzen | ||
| * | ||
| * Permission is hereby granted, free of charge, to any person obtaining | ||
| * a copy of this software and associated documentation files (the | ||
| * "Software"), to deal in the Software without restriction, including | ||
| * without limitation the rights to use, copy, modify, merge, publish, | ||
| * distribute, sublicense, and/or sell copies of the Software, and to | ||
| * permit persons to whom the Software is furnished to do so, subject to | ||
| * the following conditions: | ||
| * | ||
| * The above copyright notice and this permission notice shall be | ||
| * included in all copies or substantial portions of the Software. | ||
| * | ||
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, | ||
| * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF | ||
| * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. | ||
| * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY | ||
| * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, | ||
| * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE | ||
| * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. | ||
| */ | ||
|
|
||
| #pragma once | ||
|
|
||
| #include <unistd.h> | ||
| #include <sys/syscall.h> | ||
| #include <linux/futex.h> | ||
|
|
||
| // Implementation based on https://eli.thegreenplace.net/2018/basics-of-futexes/ and | ||
| // "Futexes are Tricky" by Ulrich Drepper. | ||
| // Kind of overkill, but we need a mutex which can work cross-process and cross-architecture via shared memory (32-bit and 64-bit). | ||
| // Alternative is full kernel semaphores or raw spinlocks. | ||
|
|
||
| namespace Fossilize | ||
| { | ||
| static inline int cmpxchg(int *value, int expected_value, int new_value) | ||
| { | ||
| int ret = __sync_val_compare_and_swap(value, expected_value, new_value); | ||
| return ret; | ||
| } | ||
|
|
||
| static inline void futex_wrapper_lock(int *lock) | ||
| { | ||
| int c = cmpxchg(lock, 0, 1); | ||
| if (c != 0) | ||
| { | ||
| // Contention. | ||
| do | ||
| { | ||
| // Need to lock. Force *lock to be 2. | ||
| if (c == 2 || cmpxchg(lock, 1, 2) != 0) | ||
| { | ||
| // If *lock is 2 (was not unlocked somehow by other thread), | ||
| // wait until it's woken up. | ||
| syscall(SYS_futex, lock, FUTEX_WAIT, 2, 0, 0, 0); | ||
| } | ||
| } while ((c = cmpxchg(lock, 0, 2)) != 0); | ||
| } | ||
| } | ||
|
|
||
| static inline void futex_wrapper_unlock(int *lock) | ||
| { | ||
| int c = __sync_sub_and_fetch(lock, 1); | ||
| if (c == 1) | ||
| { | ||
| // We have some waiters to wake up. | ||
|
|
||
| // Atomic store, really, but there's no __sync variant for that. | ||
| __sync_fetch_and_and(lock, 0); | ||
|
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| syscall(SYS_futex, lock, FUTEX_WAKE, 1, 0, 0, 0); | ||
| } | ||
| } | ||
| } |
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