1// SPDX-License-Identifier: GPL-2.0-or-later
2
3#include <linux/plist.h>
4#include <linux/sched/signal.h>
5
6#include "futex.h"
7#include "../locking/rtmutex_common.h"
8
9/*
10 * On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an
11 * underlying rtmutex. The task which is about to be requeued could have
12 * just woken up (timeout, signal). After the wake up the task has to
13 * acquire hash bucket lock, which is held by the requeue code. As a task
14 * can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking
15 * and the hash bucket lock blocking would collide and corrupt state.
16 *
17 * On !PREEMPT_RT this is not a problem and everything could be serialized
18 * on hash bucket lock, but aside of having the benefit of common code,
19 * this allows to avoid doing the requeue when the task is already on the
20 * way out and taking the hash bucket lock of the original uaddr1 when the
21 * requeue has been completed.
22 *
23 * The following state transitions are valid:
24 *
25 * On the waiter side:
26 * Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IGNORE
27 * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT
28 *
29 * On the requeue side:
30 * Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_INPROGRESS
31 * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_DONE/LOCKED
32 * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_NONE (requeue failed)
33 * Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_DONE/LOCKED
34 * Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_IGNORE (requeue failed)
35 *
36 * The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this
37 * signals that the waiter is already on the way out. It also means that
38 * the waiter is still on the 'wait' futex, i.e. uaddr1.
39 *
40 * The waiter side signals early wakeup to the requeue side either through
41 * setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending
42 * on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately
43 * proceed to take the hash bucket lock of uaddr1. If it set state to WAIT,
44 * which means the wakeup is interleaving with a requeue in progress it has
45 * to wait for the requeue side to change the state. Either to DONE/LOCKED
46 * or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex
47 * and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by
48 * the requeue side when the requeue attempt failed via deadlock detection
49 * and therefore the waiter q is still on the uaddr1 futex.
50 */
51enum {
52 Q_REQUEUE_PI_NONE = 0,
53 Q_REQUEUE_PI_IGNORE,
54 Q_REQUEUE_PI_IN_PROGRESS,
55 Q_REQUEUE_PI_WAIT,
56 Q_REQUEUE_PI_DONE,
57 Q_REQUEUE_PI_LOCKED,
58};
59
60const struct futex_q futex_q_init = {
61 /* list gets initialized in futex_queue()*/
62 .wake = futex_wake_mark,
63 .key = FUTEX_KEY_INIT,
64 .bitset = FUTEX_BITSET_MATCH_ANY,
65 .requeue_state = ATOMIC_INIT(Q_REQUEUE_PI_NONE),
66};
67
68/**
69 * requeue_futex() - Requeue a futex_q from one hb to another
70 * @q: the futex_q to requeue
71 * @hb1: the source hash_bucket
72 * @hb2: the target hash_bucket
73 * @key2: the new key for the requeued futex_q
74 */
75static inline
76void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
77 struct futex_hash_bucket *hb2, union futex_key *key2)
78{
79
80 /*
81 * If key1 and key2 hash to the same bucket, no need to
82 * requeue.
83 */
84 if (likely(&hb1->chain != &hb2->chain)) {
85 plist_del(node: &q->list, head: &hb1->chain);
86 futex_hb_waiters_dec(hb: hb1);
87 futex_hb_waiters_inc(hb: hb2);
88 plist_add(node: &q->list, head: &hb2->chain);
89 q->lock_ptr = &hb2->lock;
90 /*
91 * hb1 and hb2 belong to the same futex_hash_bucket_private
92 * because if we managed get a reference on hb1 then it can't be
93 * replaced. Therefore we avoid put(hb1)+get(hb2) here.
94 */
95 }
96 q->key = *key2;
97}
98
99static inline bool futex_requeue_pi_prepare(struct futex_q *q,
100 struct futex_pi_state *pi_state)
101{
102 int old, new;
103
104 /*
105 * Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has
106 * already set Q_REQUEUE_PI_IGNORE to signal that requeue should
107 * ignore the waiter.
108 */
109 old = atomic_read_acquire(v: &q->requeue_state);
110 do {
111 if (old == Q_REQUEUE_PI_IGNORE)
112 return false;
113
114 /*
115 * futex_proxy_trylock_atomic() might have set it to
116 * IN_PROGRESS and a interleaved early wake to WAIT.
117 *
118 * It was considered to have an extra state for that
119 * trylock, but that would just add more conditionals
120 * all over the place for a dubious value.
121 */
122 if (old != Q_REQUEUE_PI_NONE)
123 break;
124
125 new = Q_REQUEUE_PI_IN_PROGRESS;
126 } while (!atomic_try_cmpxchg(v: &q->requeue_state, old: &old, new));
127
128 q->pi_state = pi_state;
129 return true;
130}
131
132static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
133{
134 int old, new;
135
136 old = atomic_read_acquire(v: &q->requeue_state);
137 do {
138 if (old == Q_REQUEUE_PI_IGNORE)
139 return;
140
141 if (locked >= 0) {
142 /* Requeue succeeded. Set DONE or LOCKED */
143 WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
144 old != Q_REQUEUE_PI_WAIT);
145 new = Q_REQUEUE_PI_DONE + locked;
146 } else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
147 /* Deadlock, no early wakeup interleave */
148 new = Q_REQUEUE_PI_NONE;
149 } else {
150 /* Deadlock, early wakeup interleave. */
151 WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);
152 new = Q_REQUEUE_PI_IGNORE;
153 }
154 } while (!atomic_try_cmpxchg(v: &q->requeue_state, old: &old, new));
155
156#ifdef CONFIG_PREEMPT_RT
157 /* If the waiter interleaved with the requeue let it know */
158 if (unlikely(old == Q_REQUEUE_PI_WAIT))
159 rcuwait_wake_up(&q->requeue_wait);
160#endif
161}
162
163static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
164{
165 int old, new;
166
167 old = atomic_read_acquire(v: &q->requeue_state);
168 do {
169 /* Is requeue done already? */
170 if (old >= Q_REQUEUE_PI_DONE)
171 return old;
172
173 /*
174 * If not done, then tell the requeue code to either ignore
175 * the waiter or to wake it up once the requeue is done.
176 */
177 new = Q_REQUEUE_PI_WAIT;
178 if (old == Q_REQUEUE_PI_NONE)
179 new = Q_REQUEUE_PI_IGNORE;
180 } while (!atomic_try_cmpxchg(v: &q->requeue_state, old: &old, new));
181
182 /* If the requeue was in progress, wait for it to complete */
183 if (old == Q_REQUEUE_PI_IN_PROGRESS) {
184#ifdef CONFIG_PREEMPT_RT
185 rcuwait_wait_event(&q->requeue_wait,
186 atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
187 TASK_UNINTERRUPTIBLE);
188#else
189 (void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
190#endif
191 }
192
193 /*
194 * Requeue is now either prohibited or complete. Reread state
195 * because during the wait above it might have changed. Nothing
196 * will modify q->requeue_state after this point.
197 */
198 return atomic_read(v: &q->requeue_state);
199}
200
201/**
202 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
203 * @q: the futex_q
204 * @key: the key of the requeue target futex
205 * @hb: the hash_bucket of the requeue target futex
206 *
207 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
208 * target futex if it is uncontended or via a lock steal.
209 *
210 * 1) Set @q::key to the requeue target futex key so the waiter can detect
211 * the wakeup on the right futex.
212 *
213 * 2) Dequeue @q from the hash bucket.
214 *
215 * 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
216 * acquisition.
217 *
218 * 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
219 * the waiter has to fixup the pi state.
220 *
221 * 5) Complete the requeue state so the waiter can make progress. After
222 * this point the waiter task can return from the syscall immediately in
223 * case that the pi state does not have to be fixed up.
224 *
225 * 6) Wake the waiter task.
226 *
227 * Must be called with both q->lock_ptr and hb->lock held.
228 */
229static inline
230void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
231 struct futex_hash_bucket *hb)
232{
233 struct task_struct *task;
234
235 q->key = *key;
236 __futex_unqueue(q);
237
238 WARN_ON(!q->rt_waiter);
239 q->rt_waiter = NULL;
240 /*
241 * Acquire a reference for the waiter to ensure valid
242 * futex_q::lock_ptr.
243 */
244 futex_hash_get(hb);
245 q->drop_hb_ref = true;
246 q->lock_ptr = &hb->lock;
247 task = READ_ONCE(q->task);
248
249 /* Signal locked state to the waiter */
250 futex_requeue_pi_complete(q, locked: 1);
251 wake_up_state(tsk: task, TASK_NORMAL);
252}
253
254/**
255 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
256 * @pifutex: the user address of the to futex
257 * @hb1: the from futex hash bucket, must be locked by the caller
258 * @hb2: the to futex hash bucket, must be locked by the caller
259 * @key1: the from futex key
260 * @key2: the to futex key
261 * @ps: address to store the pi_state pointer
262 * @exiting: Pointer to store the task pointer of the owner task
263 * which is in the middle of exiting
264 * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
265 *
266 * Try and get the lock on behalf of the top waiter if we can do it atomically.
267 * Wake the top waiter if we succeed. If the caller specified set_waiters,
268 * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
269 * hb1 and hb2 must be held by the caller.
270 *
271 * @exiting is only set when the return value is -EBUSY. If so, this holds
272 * a refcount on the exiting task on return and the caller needs to drop it
273 * after waiting for the exit to complete.
274 *
275 * Return:
276 * - 0 - failed to acquire the lock atomically;
277 * - >0 - acquired the lock, return value is vpid of the top_waiter
278 * - <0 - error
279 */
280static int
281futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
282 struct futex_hash_bucket *hb2, union futex_key *key1,
283 union futex_key *key2, struct futex_pi_state **ps,
284 struct task_struct **exiting, int set_waiters)
285{
286 struct futex_q *top_waiter;
287 u32 curval;
288 int ret;
289
290 if (futex_get_value_locked(dest: &curval, from: pifutex))
291 return -EFAULT;
292
293 if (unlikely(should_fail_futex(true)))
294 return -EFAULT;
295
296 /*
297 * Find the top_waiter and determine if there are additional waiters.
298 * If the caller intends to requeue more than 1 waiter to pifutex,
299 * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
300 * as we have means to handle the possible fault. If not, don't set
301 * the bit unnecessarily as it will force the subsequent unlock to enter
302 * the kernel.
303 */
304 top_waiter = futex_top_waiter(hb: hb1, key: key1);
305
306 /* There are no waiters, nothing for us to do. */
307 if (!top_waiter)
308 return 0;
309
310 /*
311 * Ensure that this is a waiter sitting in futex_wait_requeue_pi()
312 * and waiting on the 'waitqueue' futex which is always !PI.
313 */
314 if (!top_waiter->rt_waiter || top_waiter->pi_state)
315 return -EINVAL;
316
317 /* Ensure we requeue to the expected futex. */
318 if (!futex_match(key1: top_waiter->requeue_pi_key, key2))
319 return -EINVAL;
320
321 /* Ensure that this does not race against an early wakeup */
322 if (!futex_requeue_pi_prepare(q: top_waiter, NULL))
323 return -EAGAIN;
324
325 /*
326 * Try to take the lock for top_waiter and set the FUTEX_WAITERS bit
327 * in the contended case or if @set_waiters is true.
328 *
329 * In the contended case PI state is attached to the lock owner. If
330 * the user space lock can be acquired then PI state is attached to
331 * the new owner (@top_waiter->task) when @set_waiters is true.
332 */
333 ret = futex_lock_pi_atomic(uaddr: pifutex, hb: hb2, key: key2, ps, task: top_waiter->task,
334 exiting, set_waiters);
335 if (ret == 1) {
336 /*
337 * Lock was acquired in user space and PI state was
338 * attached to @top_waiter->task. That means state is fully
339 * consistent and the waiter can return to user space
340 * immediately after the wakeup.
341 */
342 requeue_pi_wake_futex(q: top_waiter, key: key2, hb: hb2);
343 } else if (ret < 0) {
344 /* Rewind top_waiter::requeue_state */
345 futex_requeue_pi_complete(q: top_waiter, locked: ret);
346 } else {
347 /*
348 * futex_lock_pi_atomic() did not acquire the user space
349 * futex, but managed to establish the proxy lock and pi
350 * state. top_waiter::requeue_state cannot be fixed up here
351 * because the waiter is not enqueued on the rtmutex
352 * yet. This is handled at the callsite depending on the
353 * result of rt_mutex_start_proxy_lock() which is
354 * guaranteed to be reached with this function returning 0.
355 */
356 }
357 return ret;
358}
359
360/**
361 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
362 * @uaddr1: source futex user address
363 * @flags1: futex flags (FLAGS_SHARED, etc.)
364 * @uaddr2: target futex user address
365 * @flags2: futex flags (FLAGS_SHARED, etc.)
366 * @nr_wake: number of waiters to wake (must be 1 for requeue_pi)
367 * @nr_requeue: number of waiters to requeue (0-INT_MAX)
368 * @cmpval: @uaddr1 expected value (or %NULL)
369 * @requeue_pi: if we are attempting to requeue from a non-pi futex to a
370 * pi futex (pi to pi requeue is not supported)
371 *
372 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
373 * uaddr2 atomically on behalf of the top waiter.
374 *
375 * Return:
376 * - >=0 - on success, the number of tasks requeued or woken;
377 * - <0 - on error
378 */
379int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
380 u32 __user *uaddr2, unsigned int flags2,
381 int nr_wake, int nr_requeue, u32 *cmpval, int requeue_pi)
382{
383 union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
384 int task_count = 0, ret;
385 struct futex_pi_state *pi_state = NULL;
386 struct futex_q *this, *next;
387 DEFINE_WAKE_Q(wake_q);
388
389 if (nr_wake < 0 || nr_requeue < 0)
390 return -EINVAL;
391
392 /*
393 * When PI not supported: return -ENOSYS if requeue_pi is true,
394 * consequently the compiler knows requeue_pi is always false past
395 * this point which will optimize away all the conditional code
396 * further down.
397 */
398 if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
399 return -ENOSYS;
400
401 if (requeue_pi) {
402 /*
403 * Requeue PI only works on two distinct uaddrs. This
404 * check is only valid for private futexes. See below.
405 */
406 if (uaddr1 == uaddr2)
407 return -EINVAL;
408
409 /*
410 * futex_requeue() allows the caller to define the number
411 * of waiters to wake up via the @nr_wake argument. With
412 * REQUEUE_PI, waking up more than one waiter is creating
413 * more problems than it solves. Waking up a waiter makes
414 * only sense if the PI futex @uaddr2 is uncontended as
415 * this allows the requeue code to acquire the futex
416 * @uaddr2 before waking the waiter. The waiter can then
417 * return to user space without further action. A secondary
418 * wakeup would just make the futex_wait_requeue_pi()
419 * handling more complex, because that code would have to
420 * look up pi_state and do more or less all the handling
421 * which the requeue code has to do for the to be requeued
422 * waiters. So restrict the number of waiters to wake to
423 * one, and only wake it up when the PI futex is
424 * uncontended. Otherwise requeue it and let the unlock of
425 * the PI futex handle the wakeup.
426 *
427 * All REQUEUE_PI users, e.g. pthread_cond_signal() and
428 * pthread_cond_broadcast() must use nr_wake=1.
429 */
430 if (nr_wake != 1)
431 return -EINVAL;
432
433 /*
434 * requeue_pi requires a pi_state, try to allocate it now
435 * without any locks in case it fails.
436 */
437 if (refill_pi_state_cache())
438 return -ENOMEM;
439 }
440
441retry:
442 ret = get_futex_key(uaddr: uaddr1, flags: flags1, key: &key1, rw: FUTEX_READ);
443 if (unlikely(ret != 0))
444 return ret;
445 ret = get_futex_key(uaddr: uaddr2, flags: flags2, key: &key2,
446 rw: requeue_pi ? FUTEX_WRITE : FUTEX_READ);
447 if (unlikely(ret != 0))
448 return ret;
449
450 /*
451 * The check above which compares uaddrs is not sufficient for
452 * shared futexes. We need to compare the keys:
453 */
454 if (requeue_pi && futex_match(key1: &key1, key2: &key2))
455 return -EINVAL;
456
457retry_private:
458 if (1) {
459 CLASS(hb, hb1)(key: &key1);
460 CLASS(hb, hb2)(key: &key2);
461
462 futex_hb_waiters_inc(hb: hb2);
463 double_lock_hb(hb1, hb2);
464
465 if (likely(cmpval != NULL)) {
466 u32 curval;
467
468 ret = futex_get_value_locked(dest: &curval, from: uaddr1);
469
470 if (unlikely(ret)) {
471 futex_hb_waiters_dec(hb: hb2);
472 double_unlock_hb(hb1, hb2);
473
474 ret = get_user(curval, uaddr1);
475 if (ret)
476 return ret;
477
478 if (!(flags1 & FLAGS_SHARED))
479 goto retry_private;
480
481 goto retry;
482 }
483 if (curval != *cmpval) {
484 ret = -EAGAIN;
485 goto out_unlock;
486 }
487 }
488
489 if (requeue_pi) {
490 struct task_struct *exiting = NULL;
491
492 /*
493 * Attempt to acquire uaddr2 and wake the top waiter. If we
494 * intend to requeue waiters, force setting the FUTEX_WAITERS
495 * bit. We force this here where we are able to easily handle
496 * faults rather in the requeue loop below.
497 *
498 * Updates topwaiter::requeue_state if a top waiter exists.
499 */
500 ret = futex_proxy_trylock_atomic(pifutex: uaddr2, hb1, hb2, key1: &key1,
501 key2: &key2, ps: &pi_state,
502 exiting: &exiting, set_waiters: nr_requeue);
503
504 /*
505 * At this point the top_waiter has either taken uaddr2 or
506 * is waiting on it. In both cases pi_state has been
507 * established and an initial refcount on it. In case of an
508 * error there's nothing.
509 *
510 * The top waiter's requeue_state is up to date:
511 *
512 * - If the lock was acquired atomically (ret == 1), then
513 * the state is Q_REQUEUE_PI_LOCKED.
514 *
515 * The top waiter has been dequeued and woken up and can
516 * return to user space immediately. The kernel/user
517 * space state is consistent. In case that there must be
518 * more waiters requeued the WAITERS bit in the user
519 * space futex is set so the top waiter task has to go
520 * into the syscall slowpath to unlock the futex. This
521 * will block until this requeue operation has been
522 * completed and the hash bucket locks have been
523 * dropped.
524 *
525 * - If the trylock failed with an error (ret < 0) then
526 * the state is either Q_REQUEUE_PI_NONE, i.e. "nothing
527 * happened", or Q_REQUEUE_PI_IGNORE when there was an
528 * interleaved early wakeup.
529 *
530 * - If the trylock did not succeed (ret == 0) then the
531 * state is either Q_REQUEUE_PI_IN_PROGRESS or
532 * Q_REQUEUE_PI_WAIT if an early wakeup interleaved.
533 * This will be cleaned up in the loop below, which
534 * cannot fail because futex_proxy_trylock_atomic() did
535 * the same sanity checks for requeue_pi as the loop
536 * below does.
537 */
538 switch (ret) {
539 case 0:
540 /* We hold a reference on the pi state. */
541 break;
542
543 case 1:
544 /*
545 * futex_proxy_trylock_atomic() acquired the user space
546 * futex. Adjust task_count.
547 */
548 task_count++;
549 ret = 0;
550 break;
551
552 /*
553 * If the above failed, then pi_state is NULL and
554 * waiter::requeue_state is correct.
555 */
556 case -EFAULT:
557 futex_hb_waiters_dec(hb: hb2);
558 double_unlock_hb(hb1, hb2);
559 ret = fault_in_user_writeable(uaddr: uaddr2);
560 if (!ret)
561 goto retry;
562 return ret;
563 case -EBUSY:
564 case -EAGAIN:
565 /*
566 * Two reasons for this:
567 * - EBUSY: Owner is exiting and we just wait for the
568 * exit to complete.
569 * - EAGAIN: The user space value changed.
570 */
571 futex_hb_waiters_dec(hb: hb2);
572 double_unlock_hb(hb1, hb2);
573 /*
574 * Handle the case where the owner is in the middle of
575 * exiting. Wait for the exit to complete otherwise
576 * this task might loop forever, aka. live lock.
577 */
578 wait_for_owner_exiting(ret, exiting);
579 cond_resched();
580 goto retry;
581 default:
582 goto out_unlock;
583 }
584 }
585
586 plist_for_each_entry_safe(this, next, &hb1->chain, list) {
587 if (task_count - nr_wake >= nr_requeue)
588 break;
589
590 if (!futex_match(key1: &this->key, key2: &key1))
591 continue;
592
593 /*
594 * FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI should always
595 * be paired with each other and no other futex ops.
596 *
597 * We should never be requeueing a futex_q with a pi_state,
598 * which is awaiting a futex_unlock_pi().
599 */
600 if ((requeue_pi && !this->rt_waiter) ||
601 (!requeue_pi && this->rt_waiter) ||
602 this->pi_state) {
603 ret = -EINVAL;
604 break;
605 }
606
607 /* Plain futexes just wake or requeue and are done */
608 if (!requeue_pi) {
609 if (++task_count <= nr_wake)
610 this->wake(&wake_q, this);
611 else
612 requeue_futex(q: this, hb1, hb2, key2: &key2);
613 continue;
614 }
615
616 /* Ensure we requeue to the expected futex for requeue_pi. */
617 if (!futex_match(key1: this->requeue_pi_key, key2: &key2)) {
618 ret = -EINVAL;
619 break;
620 }
621
622 /*
623 * Requeue nr_requeue waiters and possibly one more in the case
624 * of requeue_pi if we couldn't acquire the lock atomically.
625 *
626 * Prepare the waiter to take the rt_mutex. Take a refcount
627 * on the pi_state and store the pointer in the futex_q
628 * object of the waiter.
629 */
630 get_pi_state(pi_state);
631
632 /* Don't requeue when the waiter is already on the way out. */
633 if (!futex_requeue_pi_prepare(q: this, pi_state)) {
634 /*
635 * Early woken waiter signaled that it is on the
636 * way out. Drop the pi_state reference and try the
637 * next waiter. @this->pi_state is still NULL.
638 */
639 put_pi_state(pi_state);
640 continue;
641 }
642
643 ret = rt_mutex_start_proxy_lock(lock: &pi_state->pi_mutex,
644 waiter: this->rt_waiter,
645 task: this->task);
646
647 if (ret == 1) {
648 /*
649 * We got the lock. We do neither drop the refcount
650 * on pi_state nor clear this->pi_state because the
651 * waiter needs the pi_state for cleaning up the
652 * user space value. It will drop the refcount
653 * after doing so. this::requeue_state is updated
654 * in the wakeup as well.
655 */
656 requeue_pi_wake_futex(q: this, key: &key2, hb: hb2);
657 task_count++;
658 } else if (!ret) {
659 /* Waiter is queued, move it to hb2 */
660 requeue_futex(q: this, hb1, hb2, key2: &key2);
661 futex_requeue_pi_complete(q: this, locked: 0);
662 task_count++;
663 } else {
664 /*
665 * rt_mutex_start_proxy_lock() detected a potential
666 * deadlock when we tried to queue that waiter.
667 * Drop the pi_state reference which we took above
668 * and remove the pointer to the state from the
669 * waiters futex_q object.
670 */
671 this->pi_state = NULL;
672 put_pi_state(pi_state);
673 futex_requeue_pi_complete(q: this, locked: ret);
674 /*
675 * We stop queueing more waiters and let user space
676 * deal with the mess.
677 */
678 break;
679 }
680 }
681
682 /*
683 * We took an extra initial reference to the pi_state in
684 * futex_proxy_trylock_atomic(). We need to drop it here again.
685 */
686 put_pi_state(pi_state);
687
688out_unlock:
689 futex_hb_waiters_dec(hb: hb2);
690 double_unlock_hb(hb1, hb2);
691 }
692 wake_up_q(head: &wake_q);
693 return ret ? ret : task_count;
694}
695
696/**
697 * handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
698 * @hb: the hash_bucket futex_q was original enqueued on
699 * @q: the futex_q woken while waiting to be requeued
700 * @timeout: the timeout associated with the wait (NULL if none)
701 *
702 * Determine the cause for the early wakeup.
703 *
704 * Return:
705 * -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
706 */
707static inline
708int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
709 struct futex_q *q,
710 struct hrtimer_sleeper *timeout)
711{
712 int ret;
713
714 /*
715 * With the hb lock held, we avoid races while we process the wakeup.
716 * We only need to hold hb (and not hb2) to ensure atomicity as the
717 * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
718 * It can't be requeued from uaddr2 to something else since we don't
719 * support a PI aware source futex for requeue.
720 */
721 WARN_ON_ONCE(&hb->lock != q->lock_ptr);
722
723 /*
724 * We were woken prior to requeue by a timeout or a signal.
725 * Unqueue the futex_q and determine which it was.
726 */
727 plist_del(node: &q->list, head: &hb->chain);
728 futex_hb_waiters_dec(hb);
729
730 /* Handle spurious wakeups gracefully */
731 ret = -EWOULDBLOCK;
732 if (timeout && !timeout->task)
733 ret = -ETIMEDOUT;
734 else if (signal_pending(current))
735 ret = -ERESTARTNOINTR;
736 return ret;
737}
738
739/**
740 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
741 * @uaddr: the futex we initially wait on (non-pi)
742 * @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
743 * the same type, no requeueing from private to shared, etc.
744 * @val: the expected value of uaddr
745 * @abs_time: absolute timeout
746 * @bitset: 32 bit wakeup bitset set by userspace, defaults to all
747 * @uaddr2: the pi futex we will take prior to returning to user-space
748 *
749 * The caller will wait on uaddr and will be requeued by futex_requeue() to
750 * uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake
751 * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
752 * userspace. This ensures the rt_mutex maintains an owner when it has waiters;
753 * without one, the pi logic would not know which task to boost/deboost, if
754 * there was a need to.
755 *
756 * We call schedule in futex_wait_queue() when we enqueue and return there
757 * via the following--
758 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
759 * 2) wakeup on uaddr2 after a requeue
760 * 3) signal
761 * 4) timeout
762 *
763 * If 3, cleanup and return -ERESTARTNOINTR.
764 *
765 * If 2, we may then block on trying to take the rt_mutex and return via:
766 * 5) successful lock
767 * 6) signal
768 * 7) timeout
769 * 8) other lock acquisition failure
770 *
771 * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
772 *
773 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
774 *
775 * Return:
776 * - 0 - On success;
777 * - <0 - On error
778 */
779int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
780 u32 val, ktime_t *abs_time, u32 bitset,
781 u32 __user *uaddr2)
782{
783 struct hrtimer_sleeper timeout, *to;
784 struct rt_mutex_waiter rt_waiter;
785 union futex_key key2 = FUTEX_KEY_INIT;
786 struct futex_q q = futex_q_init;
787 struct rt_mutex_base *pi_mutex;
788 int res, ret;
789
790 if (!IS_ENABLED(CONFIG_FUTEX_PI))
791 return -ENOSYS;
792
793 if (uaddr == uaddr2)
794 return -EINVAL;
795
796 if (!bitset)
797 return -EINVAL;
798
799 to = futex_setup_timer(time: abs_time, timeout: &timeout, flags,
800 current->timer_slack_ns);
801
802 /*
803 * The waiter is allocated on our stack, manipulated by the requeue
804 * code while we sleep on uaddr.
805 */
806 rt_mutex_init_waiter(waiter: &rt_waiter);
807
808 ret = get_futex_key(uaddr: uaddr2, flags, key: &key2, rw: FUTEX_WRITE);
809 if (unlikely(ret != 0))
810 goto out;
811
812 q.bitset = bitset;
813 q.rt_waiter = &rt_waiter;
814 q.requeue_pi_key = &key2;
815
816 /*
817 * Prepare to wait on uaddr. On success, it holds hb->lock and q
818 * is initialized.
819 */
820 ret = futex_wait_setup(uaddr, val, flags, q: &q, key2: &key2, current);
821 if (ret)
822 goto out;
823
824 /* Queue the futex_q, drop the hb lock, wait for wakeup. */
825 futex_do_wait(q: &q, timeout: to);
826
827 switch (futex_requeue_pi_wakeup_sync(q: &q)) {
828 case Q_REQUEUE_PI_IGNORE:
829 {
830 CLASS(hb, hb)(key: &q.key);
831 /* The waiter is still on uaddr1 */
832 spin_lock(lock: &hb->lock);
833 ret = handle_early_requeue_pi_wakeup(hb, q: &q, timeout: to);
834 spin_unlock(lock: &hb->lock);
835 }
836 break;
837
838 case Q_REQUEUE_PI_LOCKED:
839 /* The requeue acquired the lock */
840 if (q.pi_state && (q.pi_state->owner != current)) {
841 futex_q_lockptr_lock(q: &q);
842 ret = fixup_pi_owner(uaddr: uaddr2, q: &q, locked: true);
843 /*
844 * Drop the reference to the pi state which the
845 * requeue_pi() code acquired for us.
846 */
847 put_pi_state(pi_state: q.pi_state);
848 spin_unlock(lock: q.lock_ptr);
849 /*
850 * Adjust the return value. It's either -EFAULT or
851 * success (1) but the caller expects 0 for success.
852 */
853 ret = ret < 0 ? ret : 0;
854 }
855 break;
856
857 case Q_REQUEUE_PI_DONE:
858 /* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
859 pi_mutex = &q.pi_state->pi_mutex;
860 ret = rt_mutex_wait_proxy_lock(lock: pi_mutex, to, waiter: &rt_waiter);
861
862 /*
863 * See futex_unlock_pi()'s cleanup: comment.
864 */
865 if (ret && !rt_mutex_cleanup_proxy_lock(lock: pi_mutex, waiter: &rt_waiter))
866 ret = 0;
867
868 futex_q_lockptr_lock(q: &q);
869 debug_rt_mutex_free_waiter(waiter: &rt_waiter);
870 /*
871 * Fixup the pi_state owner and possibly acquire the lock if we
872 * haven't already.
873 */
874 res = fixup_pi_owner(uaddr: uaddr2, q: &q, locked: !ret);
875 /*
876 * If fixup_pi_owner() returned an error, propagate that. If it
877 * acquired the lock, clear -ETIMEDOUT or -EINTR.
878 */
879 if (res)
880 ret = (res < 0) ? res : 0;
881
882 futex_unqueue_pi(q: &q);
883 spin_unlock(lock: q.lock_ptr);
884
885 if (ret == -EINTR) {
886 /*
887 * We've already been requeued, but cannot restart
888 * by calling futex_lock_pi() directly. We could
889 * restart this syscall, but it would detect that
890 * the user space "val" changed and return
891 * -EWOULDBLOCK. Save the overhead of the restart
892 * and return -EWOULDBLOCK directly.
893 */
894 ret = -EWOULDBLOCK;
895 }
896 break;
897 default:
898 BUG();
899 }
900 if (q.drop_hb_ref) {
901 CLASS(hb, hb)(key: &q.key);
902 /* Additional reference from requeue_pi_wake_futex() */
903 futex_hash_put(hb);
904 }
905
906out:
907 if (to) {
908 hrtimer_cancel(timer: &to->timer);
909 destroy_hrtimer_on_stack(timer: &to->timer);
910 }
911 return ret;
912}
913
914