| 1 | /* |
| 2 | * SPDX-License-Identifier: LGPL-2.1-or-later |
| 3 | * |
| 4 | * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 5 | * |
| 6 | * Userspace RCU library - sys_futex compatibility code |
| 7 | */ |
| 8 | |
| 9 | #include <stdio.h> |
| 10 | #include <pthread.h> |
| 11 | #include <signal.h> |
| 12 | #include <assert.h> |
| 13 | #include <errno.h> |
| 14 | #include <poll.h> |
| 15 | #include <stdint.h> |
| 16 | |
| 17 | #include <urcu/arch.h> |
| 18 | #include <urcu/system.h> |
| 19 | |
| 20 | #include "lib/lttng-ust/futex.h" |
| 21 | |
| 22 | /* |
| 23 | * This compat header originated in userspace RCU where it's used across |
| 24 | * multiple shared objects hence the need to have the mutexes as public weak |
| 25 | * symbols, in our case here, it's only used internally by liblttng-ust so we |
| 26 | * can hide them. If we end up using this compat header in another library in |
| 27 | * this project we will have to use the same scheme, but in the meantime, don't |
| 28 | * expose those symbols in the ABI. |
| 29 | */ |
| 30 | |
| 31 | /* |
| 32 | * This comment will apply if we start using this compat header in multiple |
| 33 | * libraires. |
| 34 | * |
| 35 | * Using attribute "weak" for __lttng_ust_compat_futex_lock and |
| 36 | * __lttng_ust_compat_futex_cond. Those are globally visible by the entire |
| 37 | * program, even though many shared objects may have their own version. |
| 38 | * The first version that gets loaded will be used by the entire program |
| 39 | * (executable and all shared objects). |
| 40 | */ |
| 41 | |
| 42 | static pthread_mutex_t __lttng_ust_compat_futex_lock = PTHREAD_MUTEX_INITIALIZER; |
| 43 | static pthread_cond_t __lttng_ust_compat_futex_cond = PTHREAD_COND_INITIALIZER; |
| 44 | |
| 45 | /* |
| 46 | * _NOT SIGNAL-SAFE_. pthread_cond is not signal-safe anyway. Though. |
| 47 | * For now, timeout, uaddr2 and val3 are unused. |
| 48 | * Waiter will relinquish the CPU until woken up. |
| 49 | */ |
| 50 | |
| 51 | int lttng_ust_compat_futex_noasync(int32_t *uaddr, int op, int32_t val, |
| 52 | const struct timespec *timeout, int32_t *uaddr2, int32_t val3) |
| 53 | { |
| 54 | int ret = 0, lockret; |
| 55 | |
| 56 | /* |
| 57 | * Check if NULL. Don't let users expect that they are taken into |
| 58 | * account. |
| 59 | */ |
| 60 | assert(!timeout); |
| 61 | assert(!uaddr2); |
| 62 | assert(!val3); |
| 63 | |
| 64 | /* |
| 65 | * memory barriers to serialize with the previous uaddr modification. |
| 66 | */ |
| 67 | cmm_smp_mb(); |
| 68 | |
| 69 | lockret = pthread_mutex_lock(&__lttng_ust_compat_futex_lock); |
| 70 | if (lockret) { |
| 71 | errno = lockret; |
| 72 | ret = -1; |
| 73 | goto end; |
| 74 | } |
| 75 | switch (op) { |
| 76 | case FUTEX_WAIT: |
| 77 | /* |
| 78 | * Wait until *uaddr is changed to something else than "val". |
| 79 | * Comparing *uaddr content against val figures out which |
| 80 | * thread has been awakened. |
| 81 | */ |
| 82 | while (CMM_LOAD_SHARED(*uaddr) == val) |
| 83 | pthread_cond_wait(&__lttng_ust_compat_futex_cond, |
| 84 | &__lttng_ust_compat_futex_lock); |
| 85 | break; |
| 86 | case FUTEX_WAKE: |
| 87 | /* |
| 88 | * Each wake is sending a broadcast, thus attempting wakeup of |
| 89 | * all awaiting threads, independently of their respective |
| 90 | * uaddr. |
| 91 | */ |
| 92 | pthread_cond_broadcast(&__lttng_ust_compat_futex_cond); |
| 93 | break; |
| 94 | default: |
| 95 | errno = EINVAL; |
| 96 | ret = -1; |
| 97 | } |
| 98 | lockret = pthread_mutex_unlock(&__lttng_ust_compat_futex_lock); |
| 99 | if (lockret) { |
| 100 | errno = lockret; |
| 101 | ret = -1; |
| 102 | } |
| 103 | end: |
| 104 | return ret; |
| 105 | } |
| 106 | |
| 107 | /* |
| 108 | * _ASYNC SIGNAL-SAFE_. |
| 109 | * For now, timeout, uaddr2 and val3 are unused. |
| 110 | * Waiter will busy-loop trying to read the condition. |
| 111 | * It is OK to use compat_futex_async() on a futex address on which |
| 112 | * futex() WAKE operations are also performed. |
| 113 | */ |
| 114 | |
| 115 | int lttng_ust_compat_futex_async(int32_t *uaddr, int op, int32_t val, |
| 116 | const struct timespec *timeout, int32_t *uaddr2, int32_t val3) |
| 117 | { |
| 118 | int ret = 0; |
| 119 | |
| 120 | /* |
| 121 | * Check if NULL. Don't let users expect that they are taken into |
| 122 | * account. |
| 123 | */ |
| 124 | assert(!timeout); |
| 125 | assert(!uaddr2); |
| 126 | assert(!val3); |
| 127 | |
| 128 | /* |
| 129 | * Ensure previous memory operations on uaddr have completed. |
| 130 | */ |
| 131 | cmm_smp_mb(); |
| 132 | |
| 133 | switch (op) { |
| 134 | case FUTEX_WAIT: |
| 135 | while (CMM_LOAD_SHARED(*uaddr) == val) { |
| 136 | if (poll(NULL, 0, 10) < 0) { |
| 137 | ret = -1; |
| 138 | /* Keep poll errno. Caller handles EINTR. */ |
| 139 | goto end; |
| 140 | } |
| 141 | } |
| 142 | break; |
| 143 | case FUTEX_WAKE: |
| 144 | break; |
| 145 | default: |
| 146 | errno = EINVAL; |
| 147 | ret = -1; |
| 148 | } |
| 149 | end: |
| 150 | return ret; |
| 151 | } |