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