1/*
2 * QEMU coroutine implementation
3 *
4 * Copyright IBM, Corp. 2011
5 *
6 * Authors:
7 * Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
8 * Kevin Wolf <kwolf@redhat.com>
9 *
10 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
11 * See the COPYING.LIB file in the top-level directory.
12 *
13 */
14
15#ifndef QEMU_COROUTINE_H
16#define QEMU_COROUTINE_H
17
18#include "qemu/queue.h"
19#include "qemu/timer.h"
20
21/**
22 * Coroutines are a mechanism for stack switching and can be used for
23 * cooperative userspace threading. These functions provide a simple but
24 * useful flavor of coroutines that is suitable for writing sequential code,
25 * rather than callbacks, for operations that need to give up control while
26 * waiting for events to complete.
27 *
28 * These functions are re-entrant and may be used outside the global mutex.
29 */
30
31/**
32 * Mark a function that executes in coroutine context
33 *
34 * Functions that execute in coroutine context cannot be called directly from
35 * normal functions. In the future it would be nice to enable compiler or
36 * static checker support for catching such errors. This annotation might make
37 * it possible and in the meantime it serves as documentation.
38 *
39 * For example:
40 *
41 * static void coroutine_fn foo(void) {
42 * ....
43 * }
44 */
45#define coroutine_fn
46
47typedef struct Coroutine Coroutine;
48
49/**
50 * Coroutine entry point
51 *
52 * When the coroutine is entered for the first time, opaque is passed in as an
53 * argument.
54 *
55 * When this function returns, the coroutine is destroyed automatically and
56 * execution continues in the caller who last entered the coroutine.
57 */
58typedef void coroutine_fn CoroutineEntry(void *opaque);
59
60/**
61 * Create a new coroutine
62 *
63 * Use qemu_coroutine_enter() to actually transfer control to the coroutine.
64 * The opaque argument is passed as the argument to the entry point.
65 */
66Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque);
67
68/**
69 * Transfer control to a coroutine
70 */
71void qemu_coroutine_enter(Coroutine *coroutine);
72
73/**
74 * Transfer control to a coroutine if it's not active (i.e. part of the call
75 * stack of the running coroutine). Otherwise, do nothing.
76 */
77void qemu_coroutine_enter_if_inactive(Coroutine *co);
78
79/**
80 * Transfer control to a coroutine and associate it with ctx
81 */
82void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co);
83
84/**
85 * Transfer control back to a coroutine's caller
86 *
87 * This function does not return until the coroutine is re-entered using
88 * qemu_coroutine_enter().
89 */
90void coroutine_fn qemu_coroutine_yield(void);
91
92/**
93 * Get the AioContext of the given coroutine
94 */
95AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
96
97/**
98 * Get the currently executing coroutine
99 */
100Coroutine *coroutine_fn qemu_coroutine_self(void);
101
102/**
103 * Return whether or not currently inside a coroutine
104 *
105 * This can be used to write functions that work both when in coroutine context
106 * and when not in coroutine context. Note that such functions cannot use the
107 * coroutine_fn annotation since they work outside coroutine context.
108 */
109bool qemu_in_coroutine(void);
110
111/**
112 * Return true if the coroutine is currently entered
113 *
114 * A coroutine is "entered" if it has not yielded from the current
115 * qemu_coroutine_enter() call used to run it. This does not mean that the
116 * coroutine is currently executing code since it may have transferred control
117 * to another coroutine using qemu_coroutine_enter().
118 *
119 * When several coroutines enter each other there may be no way to know which
120 * ones have already been entered. In such situations this function can be
121 * used to avoid recursively entering coroutines.
122 */
123bool qemu_coroutine_entered(Coroutine *co);
124
125/**
126 * Provides a mutex that can be used to synchronise coroutines
127 */
128struct CoWaitRecord;
129struct CoMutex {
130 /* Count of pending lockers; 0 for a free mutex, 1 for an
131 * uncontended mutex.
132 */
133 unsigned locked;
134
135 /* Context that is holding the lock. Useful to avoid spinning
136 * when two coroutines on the same AioContext try to get the lock. :)
137 */
138 AioContext *ctx;
139
140 /* A queue of waiters. Elements are added atomically in front of
141 * from_push. to_pop is only populated, and popped from, by whoever
142 * is in charge of the next wakeup. This can be an unlocker or,
143 * through the handoff protocol, a locker that is about to go to sleep.
144 */
145 QSLIST_HEAD(, CoWaitRecord) from_push, to_pop;
146
147 unsigned handoff, sequence;
148
149 Coroutine *holder;
150};
151
152/**
153 * Initialises a CoMutex. This must be called before any other operation is used
154 * on the CoMutex.
155 */
156void qemu_co_mutex_init(CoMutex *mutex);
157
158/**
159 * Locks the mutex. If the lock cannot be taken immediately, control is
160 * transferred to the caller of the current coroutine.
161 */
162void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex);
163
164/**
165 * Unlocks the mutex and schedules the next coroutine that was waiting for this
166 * lock to be run.
167 */
168void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex);
169
170
171/**
172 * CoQueues are a mechanism to queue coroutines in order to continue executing
173 * them later. They are similar to condition variables, but they need help
174 * from an external mutex in order to maintain thread-safety.
175 */
176typedef struct CoQueue {
177 QSIMPLEQ_HEAD(, Coroutine) entries;
178} CoQueue;
179
180/**
181 * Initialise a CoQueue. This must be called before any other operation is used
182 * on the CoQueue.
183 */
184void qemu_co_queue_init(CoQueue *queue);
185
186/**
187 * Adds the current coroutine to the CoQueue and transfers control to the
188 * caller of the coroutine. The mutex is unlocked during the wait and
189 * locked again afterwards.
190 */
191#define qemu_co_queue_wait(queue, lock) \
192 qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock))
193void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
194
195/**
196 * Removes the next coroutine from the CoQueue, and wake it up.
197 * Returns true if a coroutine was removed, false if the queue is empty.
198 */
199bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
200
201/**
202 * Empties the CoQueue; all coroutines are woken up.
203 */
204void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
205
206/**
207 * Removes the next coroutine from the CoQueue, and wake it up. Unlike
208 * qemu_co_queue_next, this function releases the lock during aio_co_wake
209 * because it is meant to be used outside coroutine context; in that case, the
210 * coroutine is entered immediately, before qemu_co_enter_next returns.
211 *
212 * If used in coroutine context, qemu_co_enter_next is equivalent to
213 * qemu_co_queue_next.
214 */
215#define qemu_co_enter_next(queue, lock) \
216 qemu_co_enter_next_impl(queue, QEMU_MAKE_LOCKABLE(lock))
217bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock);
218
219/**
220 * Checks if the CoQueue is empty.
221 */
222bool qemu_co_queue_empty(CoQueue *queue);
223
224
225typedef struct CoRwlock {
226 int pending_writer;
227 int reader;
228 CoMutex mutex;
229 CoQueue queue;
230} CoRwlock;
231
232/**
233 * Initialises a CoRwlock. This must be called before any other operation
234 * is used on the CoRwlock
235 */
236void qemu_co_rwlock_init(CoRwlock *lock);
237
238/**
239 * Read locks the CoRwlock. If the lock cannot be taken immediately because
240 * of a parallel writer, control is transferred to the caller of the current
241 * coroutine.
242 */
243void qemu_co_rwlock_rdlock(CoRwlock *lock);
244
245/**
246 * Write Locks the CoRwlock from a reader. This is a bit more efficient than
247 * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
248 * However, if the lock cannot be upgraded immediately, control is transferred
249 * to the caller of the current coroutine. Also, @qemu_co_rwlock_upgrade
250 * only overrides CoRwlock fairness if there are no concurrent readers, so
251 * another writer might run while @qemu_co_rwlock_upgrade blocks.
252 */
253void qemu_co_rwlock_upgrade(CoRwlock *lock);
254
255/**
256 * Downgrades a write-side critical section to a reader. Downgrading with
257 * @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
258 * followed by @qemu_co_rwlock_rdlock. This makes it more efficient, but
259 * may also sometimes be necessary for correctness.
260 */
261void qemu_co_rwlock_downgrade(CoRwlock *lock);
262
263/**
264 * Write Locks the mutex. If the lock cannot be taken immediately because
265 * of a parallel reader, control is transferred to the caller of the current
266 * coroutine.
267 */
268void qemu_co_rwlock_wrlock(CoRwlock *lock);
269
270/**
271 * Unlocks the read/write lock and schedules the next coroutine that was
272 * waiting for this lock to be run.
273 */
274void qemu_co_rwlock_unlock(CoRwlock *lock);
275
276/**
277 * Yield the coroutine for a given duration
278 */
279void coroutine_fn qemu_co_sleep_ns(QEMUClockType type, int64_t ns);
280
281/**
282 * Yield until a file descriptor becomes readable
283 *
284 * Note that this function clobbers the handlers for the file descriptor.
285 */
286void coroutine_fn yield_until_fd_readable(int fd);
287
288#include "qemu/lockable.h"
289
290#endif /* QEMU_COROUTINE_H */
291