1 /* SPDX-License-Identifier: GPL-2.0-or-later */
8  * - FREE -> SET (qemu_event_set)
9  * - BUSY -> SET (qemu_event_set)
10  * - SET -> FREE (qemu_event_reset)
11  * - FREE -> BUSY (qemu_event_wait)
14  * BUSY -> SET and FREE -> BUSY, respectively.
16  * Without futex, BUSY -> SET and FREE -> BUSY never happen. Instead, the waking
17  * operation follows FREE -> SET and the blocking operation will happen in
18  * qemu_event_wait() if the event is not SET.
20  * SET->BUSY does not happen (it can be observed from the outside but
21  * it really is SET->FREE->BUSY).
23  * busy->free provably cannot happen; to enforce it, the set->free transition
24  * is done with an OR, which becomes a no-op if the event has concurrently
30 #define EV_BUSY       -1
35     pthread_mutex_init(&ev->lock, NULL);  in qemu_event_init()
36     pthread_cond_init(&ev->cond, NULL);  in qemu_event_init()
39     ev->value = (init ? EV_SET : EV_FREE);  in qemu_event_init()
40     ev->initialized = true;  in qemu_event_init()
45     assert(ev->initialized);  in qemu_event_destroy()
46     ev->initialized = false;  in qemu_event_destroy()
48     pthread_mutex_destroy(&ev->lock);  in qemu_event_destroy()
49     pthread_cond_destroy(&ev->cond);  in qemu_event_destroy()
55     assert(ev->initialized);  in qemu_event_set()
62      * ev->value we need a full memory barrier here.  in qemu_event_set()
65     if (qatomic_read(&ev->value) != EV_SET) {  in qemu_event_set()
66         int old = qatomic_xchg(&ev->value, EV_SET);  in qemu_event_set()
76     pthread_mutex_lock(&ev->lock);  in qemu_event_set()
78     qatomic_store_release(&ev->value, EV_SET);  in qemu_event_set()
79     pthread_cond_broadcast(&ev->cond);  in qemu_event_set()
80     pthread_mutex_unlock(&ev->lock);  in qemu_event_set()
86     assert(ev->initialized);  in qemu_event_reset()
91      * do nothing.  Otherwise change EV_SET->EV_FREE.  in qemu_event_reset()
93     qatomic_or(&ev->value, EV_FREE);  in qemu_event_reset()
102      * If futexes are not available, there are no EV_FREE->EV_BUSY  in qemu_event_reset()
113      *   ev->value = EV_SET ----->     load ev->value  in qemu_event_reset()
114      *                                 ev->value = old value | EV_FREE  in qemu_event_reset()
120     qatomic_set(&ev->value, qatomic_load_acquire(&ev->value) | EV_FREE);  in qemu_event_reset()
126     assert(ev->initialized);  in qemu_event_wait()
132          * not go down the slow path, so this load-acquire is needed that  in qemu_event_wait()
135         unsigned value = qatomic_load_acquire(&ev->value);  in qemu_event_wait()
144              * busy->free transition.  After the CAS, the event will be either  in qemu_event_wait()
145              * set or busy.  in qemu_event_wait()
152             if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {  in qemu_event_wait()
158          * This is the final check for a concurrent set, so it does need  in qemu_event_wait()
165     pthread_mutex_lock(&ev->lock);  in qemu_event_wait()
166     while (qatomic_read(&ev->value) != EV_SET) {  in qemu_event_wait()
167         pthread_cond_wait(&ev->cond, &ev->lock);  in qemu_event_wait()
169     pthread_mutex_unlock(&ev->lock);  in qemu_event_wait()