2 * Copyright (C) 2011 EfficiOS Inc.
3 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
5 * SPDX-License-Identifier: GPL-2.0-only
12 #include <common/common.hpp>
17 #include <urcu/futex.h>
20 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
21 * "nto1" added to all function signature.
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.
27 * There is also a formal verification available in the git tree.
29 * branch: formal-model
30 * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
32 * Ref: git://git.lttng.org/userspace-rcu.git
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).
41 void futex_wait_update(int32_t *futex
, int active
)
44 uatomic_set(futex
, 1);
45 if (futex_async(futex
, FUTEX_WAKE
, INT_MAX
, nullptr, nullptr, 0) < 0) {
46 PERROR("futex_async");
50 uatomic_set(futex
, 0);
53 DBG("Futex wait update active %d", active
);
59 void futex_nto1_prepare(int32_t *futex
)
61 uatomic_set(futex
, -1);
64 DBG("Futex n to 1 prepare done");
70 void futex_nto1_wait(int32_t *futex
)
74 while (uatomic_read(futex
) == -1) {
75 if (!futex_async(futex
, FUTEX_WAIT
, -1, nullptr, nullptr, 0)) {
77 * Prior queued wakeups queued by unrelated code
78 * using the same address can cause futex wait to
79 * return 0 even through the futex value is still
80 * -1 (spurious wakeups). Check the value again
81 * in user-space to validate whether it really
88 /* Value already changed. */
91 /* Retry if interrupted by signal. */
92 break; /* Get out of switch. Check again. */
94 /* Unexpected error. */
95 PERROR("futex_async");
100 DBG("Futex n to 1 wait done");
106 void futex_nto1_wake(int32_t *futex
)
108 if (caa_unlikely(uatomic_read(futex
) != -1))
110 uatomic_set(futex
, 0);
111 if (futex_async(futex
, FUTEX_WAKE
, 1, nullptr, nullptr, 0) < 0) {
112 PERROR("futex_async");
116 DBG("Futex n to 1 wake done");
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