| 1 | /* |
| 2 | * Copyright (C) 2011 EfficiOS Inc. |
| 3 | * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 4 | * |
| 5 | * SPDX-License-Identifier: GPL-2.0-only |
| 6 | * |
| 7 | */ |
| 8 | |
| 9 | #define _LGPL_SOURCE |
| 10 | #include <limits.h> |
| 11 | #include <unistd.h> |
| 12 | #include <urcu.h> |
| 13 | #include <urcu/futex.h> |
| 14 | |
| 15 | #include <common/common.h> |
| 16 | |
| 17 | #include "futex.h" |
| 18 | |
| 19 | /* |
| 20 | * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the |
| 21 | * "nto1" added to all function signature. |
| 22 | * |
| 23 | * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu |
| 24 | * git tree for a detail example of this scheme being used. futex_async() is |
| 25 | * the urcu wrapper over the futex() sycall. |
| 26 | * |
| 27 | * There is also a formal verification available in the git tree. |
| 28 | * |
| 29 | * branch: formal-model |
| 30 | * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a |
| 31 | * |
| 32 | * Ref: git://git.lttng.org/userspace-rcu.git |
| 33 | */ |
| 34 | |
| 35 | /* |
| 36 | * Update futex according to active or not. This scheme is used to wake every |
| 37 | * libust waiting on the shared memory map futex hence the INT_MAX used in the |
| 38 | * futex() call. If active, we set the value and wake everyone else we indicate |
| 39 | * that we are gone (cleanup() case). |
| 40 | */ |
| 41 | void futex_wait_update(int32_t *futex, int active) |
| 42 | { |
| 43 | if (active) { |
| 44 | uatomic_set(futex, 1); |
| 45 | if (futex_async(futex, FUTEX_WAKE, |
| 46 | INT_MAX, NULL, NULL, 0) < 0) { |
| 47 | PERROR("futex_async"); |
| 48 | abort(); |
| 49 | } |
| 50 | } else { |
| 51 | uatomic_set(futex, 0); |
| 52 | } |
| 53 | |
| 54 | DBG("Futex wait update active %d", active); |
| 55 | } |
| 56 | |
| 57 | /* |
| 58 | * Prepare futex. |
| 59 | */ |
| 60 | void futex_nto1_prepare(int32_t *futex) |
| 61 | { |
| 62 | uatomic_set(futex, -1); |
| 63 | cmm_smp_mb(); |
| 64 | |
| 65 | DBG("Futex n to 1 prepare done"); |
| 66 | } |
| 67 | |
| 68 | /* |
| 69 | * Wait futex. |
| 70 | */ |
| 71 | void futex_nto1_wait(int32_t *futex) |
| 72 | { |
| 73 | cmm_smp_mb(); |
| 74 | |
| 75 | if (uatomic_read(futex) != -1) |
| 76 | goto end; |
| 77 | while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) { |
| 78 | switch (errno) { |
| 79 | case EWOULDBLOCK: |
| 80 | /* Value already changed. */ |
| 81 | goto end; |
| 82 | case EINTR: |
| 83 | /* Retry if interrupted by signal. */ |
| 84 | break; /* Get out of switch. */ |
| 85 | default: |
| 86 | /* Unexpected error. */ |
| 87 | PERROR("futex_async"); |
| 88 | abort(); |
| 89 | } |
| 90 | } |
| 91 | end: |
| 92 | DBG("Futex n to 1 wait done"); |
| 93 | } |
| 94 | |
| 95 | /* |
| 96 | * Wake 1 futex. |
| 97 | */ |
| 98 | void futex_nto1_wake(int32_t *futex) |
| 99 | { |
| 100 | if (caa_unlikely(uatomic_read(futex) != -1)) |
| 101 | goto end; |
| 102 | uatomic_set(futex, 0); |
| 103 | if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) { |
| 104 | PERROR("futex_async"); |
| 105 | abort(); |
| 106 | } |
| 107 | end: |
| 108 | DBG("Futex n to 1 wake done"); |
| 109 | } |