1 /*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
7 *
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
12 *
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
18 *
19 * CDDL HEADER END
20 */
21 /*
22 * Copyright 2008 Sun Microsystems, Inc. All rights reserved.
23 * Use is subject to license terms.
24 */
25
26 /*
27 * Copyright (c) 2013, Joyent, Inc. All rights reserved.
28 */
29
30 #include <sys/param.h>
31 #include <sys/thread.h>
32 #include <sys/cmn_err.h>
33 #include <sys/debug.h>
34 #include <sys/cpuvar.h>
35 #include <sys/sobject.h>
36 #include <sys/turnstile.h>
37 #include <sys/rwlock.h>
38 #include <sys/rwlock_impl.h>
39 #include <sys/atomic.h>
40 #include <sys/lockstat.h>
41
42 /*
43 * Big Theory Statement for readers/writer locking primitives.
44 *
45 * An rwlock provides exclusive access to a single thread ("writer") or
46 * concurrent access to multiple threads ("readers"). See rwlock(9F)
47 * for a full description of the interfaces and programming model.
48 * The rest of this comment describes the implementation.
49 *
50 * An rwlock is a single word with the following structure:
51 *
52 * ---------------------------------------------------------------------
53 * | OWNER (writer) or HOLD COUNT (readers) | WRLOCK | WRWANT | WAIT |
54 * ---------------------------------------------------------------------
55 * 63 / 31 .. 3 2 1 0
56 *
57 * The waiters bit (0) indicates whether any threads are blocked waiting
58 * for the lock. The write-wanted bit (1) indicates whether any threads
59 * are blocked waiting for write access. The write-locked bit (2) indicates
60 * whether the lock is held by a writer, which determines whether the upper
61 * bits (3..31 in ILP32, 3..63 in LP64) should be interpreted as the owner
62 * (thread pointer) or the hold count (number of readers).
63 *
64 * In the absence of any contention, a writer gets the lock by setting
65 * this word to (curthread | RW_WRITE_LOCKED); a reader gets the lock
66 * by incrementing the hold count (i.e. adding 8, aka RW_READ_LOCK).
67 *
68 * A writer will fail to acquire the lock if any other thread owns it.
69 * A reader will fail if the lock is either owned (in the RW_READER and
70 * RW_READER_STARVEWRITER cases) or wanted by a writer (in the RW_READER
71 * case). rw_tryenter() returns 0 in these cases; rw_enter() blocks until
72 * the lock becomes available.
73 *
74 * When a thread blocks it acquires the rwlock's hashed turnstile lock and
75 * attempts to set RW_HAS_WAITERS (and RW_WRITE_WANTED in the writer case)
76 * atomically *only if the lock still appears busy*. A thread must never
77 * accidentally block for an available lock since there would be no owner
78 * to awaken it. casip() provides the required atomicity. Once casip()
79 * succeeds, the decision to block becomes final and irreversible. The
80 * thread will not become runnable again until it has been granted ownership
81 * of the lock via direct handoff from a former owner as described below.
82 *
83 * In the absence of any waiters, rw_exit() just clears the lock (if it
84 * is write-locked) or decrements the hold count (if it is read-locked).
85 * Note that even if waiters are present, decrementing the hold count
86 * to a non-zero value requires no special action since the lock is still
87 * held by at least one other thread.
88 *
89 * On the "final exit" (transition to unheld state) of a lock with waiters,
90 * rw_exit_wakeup() grabs the turnstile lock and transfers ownership directly
91 * to the next writer or set of readers. There are several advantages to this
92 * approach: (1) it closes all windows for priority inversion (when a new
93 * writer has grabbed the lock but has not yet inherited from blocked readers);
94 * (2) it prevents starvation of equal-priority threads by granting the lock
95 * in FIFO order; (3) it eliminates the need for a write-wanted count -- a
96 * single bit suffices because the lock remains held until all waiting
97 * writers are gone; (4) when we awaken N readers we can perform a single
98 * "atomic_add(&x, N)" to set the total hold count rather than having all N
99 * threads fight for the cache to perform an "atomic_add(&x, 1)" upon wakeup.
100 *
101 * The most interesting policy decision in rw_exit_wakeup() is which thread
102 * to wake. Starvation is always possible with priority-based scheduling,
103 * but any sane wakeup policy should at least satisfy these requirements:
104 *
105 * (1) The highest-priority thread in the system should not starve.
106 * (2) The highest-priority writer should not starve.
107 * (3) No writer should starve due to lower-priority threads.
108 * (4) No reader should starve due to lower-priority writers.
109 * (5) If all threads have equal priority, none of them should starve.
110 *
111 * We used to employ a writers-always-win policy, which doesn't even
112 * satisfy (1): a steady stream of low-priority writers can starve out
113 * a real-time reader! This is clearly a broken policy -- it violates
114 * (1), (4), and (5) -- but it's how rwlocks always used to behave.
115 *
116 * A round-robin policy (exiting readers grant the lock to blocked writers
117 * and vice versa) satisfies all but (3): a single high-priority writer
118 * and many low-priority readers can starve out medium-priority writers.
119 *
120 * A strict priority policy (grant the lock to the highest priority blocked
121 * thread) satisfies everything but (2): a steady stream of high-priority
122 * readers can permanently starve the highest-priority writer.
123 *
124 * The reason we care about (2) is that it's important to process writers
125 * reasonably quickly -- even if they're low priority -- because their very
126 * presence causes all readers to take the slow (blocking) path through this
127 * code. There is also a general sense that writers deserve some degree of
128 * deference because they're updating the data upon which all readers act.
129 * Presumably this data should not be allowed to become arbitrarily stale
130 * due to writer starvation. Finally, it seems reasonable to level the
131 * playing field a bit to compensate for the fact that it's so much harder
132 * for a writer to get in when there are already many readers present.
133 *
134 * A hybrid of round-robin and strict priority can be made to satisfy
135 * all five criteria. In this "writer priority policy" exiting readers
136 * always grant the lock to waiting writers, but exiting writers only
137 * grant the lock to readers of the same or higher priority than the
138 * highest-priority blocked writer. Thus requirement (2) is satisfied,
139 * necessarily, by a willful act of priority inversion: an exiting reader
140 * will grant the lock to a blocked writer even if there are blocked
141 * readers of higher priority. The situation is mitigated by the fact
142 * that writers always inherit priority from blocked readers, and the
143 * writer will awaken those readers as soon as it exits the lock.
144 *
145 * Finally, note that this hybrid scheme -- and indeed, any scheme that
146 * satisfies requirement (2) -- has an important consequence: if a lock is
147 * held as reader and a writer subsequently becomes blocked, any further
148 * readers must be blocked to avoid writer starvation. This implementation
149 * detail has ramifications for the semantics of rwlocks, as it prohibits
150 * recursively acquiring an rwlock as reader: any writer that wishes to
151 * acquire the lock after the first but before the second acquisition as
152 * reader will block the second acquisition -- resulting in deadlock. This
153 * itself is not necessarily prohibitive, as it is often straightforward to
154 * prevent a single thread from recursively acquiring an rwlock as reader.
155 * However, a more subtle situation arises when both a traditional mutex and
156 * a reader lock are acquired by two different threads in opposite order.
157 * (That is, one thread first acquires the mutex and then the rwlock as
158 * reader; the other acquires the rwlock as reader and then the mutex.) As
159 * with the single threaded case, this is fine absent a blocked writer: the
160 * thread that acquires the mutex before acquiring the rwlock as reader will
161 * be able to successfully acquire the rwlock -- even as/if the other thread
162 * has the rwlock as reader and is blocked on the held mutex. However, if
163 * an unrelated writer (that is, a third thread) becomes blocked on the
164 * rwlock after the first thread acquires the rwlock as reader but before
165 * it's able to acquire the mutex, the second thread -- with the mutex held
166 * -- will not be able to acquire the rwlock as reader due to the waiting
167 * writer, deadlocking the three threads. Unlike the single-threaded
168 * (recursive) rwlock acquisition case, this case can be quite a bit
169 * thornier to fix, especially as there is nothing inherently wrong in the
170 * locking strategy: the deadlock is really induced by requirement (2), not
171 * the consumers of the rwlock. To permit such consumers, we allow rwlock
172 * acquirers to explicitly opt out of requirement (2) by specifying
173 * RW_READER_STARVEWRITER when acquiring the rwlock. This (obviously) means
174 * that inifinite readers can starve writers, but it also allows for
175 * multiple readers in the presence of other synchronization primitives
176 * without regard for lock-ordering. And while certainly odd (and perhaps
177 * unwise), RW_READER_STARVEWRITER can be safely used alongside RW_READER on
178 * the same lock -- RW_READER_STARVEWRITER describes only the act of lock
179 * acquisition with respect to waiting writers, not the lock itself.
180 *
181 * rw_downgrade() follows the same wakeup policy as an exiting writer.
182 *
183 * rw_tryupgrade() has the same failure mode as rw_tryenter() for a
184 * write lock. Both honor the WRITE_WANTED bit by specification.
185 *
186 * The following rules apply to manipulation of rwlock internal state:
187 *
188 * (1) The rwlock is only modified via the atomic primitives casip()
189 * and atomic_add_ip().
190 *
191 * (2) The waiters bit and write-wanted bit are only modified under
192 * turnstile_lookup(). This ensures that the turnstile is consistent
193 * with the rwlock.
194 *
195 * (3) Waiters receive the lock by direct handoff from the previous
196 * owner. Therefore, waiters *always* wake up holding the lock.
197 */
198
199 /*
200 * The sobj_ops vector exports a set of functions needed when a thread
201 * is asleep on a synchronization object of a given type.
202 */
203 static sobj_ops_t rw_sobj_ops = {
204 SOBJ_RWLOCK, rw_owner, turnstile_stay_asleep, turnstile_change_pri
205 };
206
207 /*
208 * If the system panics on an rwlock, save the address of the offending
209 * rwlock in panic_rwlock_addr, and save the contents in panic_rwlock.
210 */
211 static rwlock_impl_t panic_rwlock;
212 static rwlock_impl_t *panic_rwlock_addr;
213
214 static void
rw_panic(char * msg,rwlock_impl_t * lp)215 rw_panic(char *msg, rwlock_impl_t *lp)
216 {
217 if (panicstr)
218 return;
219
220 if (atomic_cas_ptr(&panic_rwlock_addr, NULL, lp) == NULL)
221 panic_rwlock = *lp;
222
223 panic("%s, lp=%p wwwh=%lx thread=%p",
224 msg, (void *)lp, panic_rwlock.rw_wwwh, (void *)curthread);
225 }
226
227 /* ARGSUSED */
228 void
rw_init(krwlock_t * rwlp,char * name,krw_type_t type,void * arg)229 rw_init(krwlock_t *rwlp, char *name, krw_type_t type, void *arg)
230 {
231 ((rwlock_impl_t *)rwlp)->rw_wwwh = 0;
232 }
233
234 void
rw_destroy(krwlock_t * rwlp)235 rw_destroy(krwlock_t *rwlp)
236 {
237 rwlock_impl_t *lp = (rwlock_impl_t *)rwlp;
238
239 if (lp->rw_wwwh != 0) {
240 if ((lp->rw_wwwh & RW_DOUBLE_LOCK) == RW_DOUBLE_LOCK)
241 rw_panic("rw_destroy: lock already destroyed", lp);
242 else
243 rw_panic("rw_destroy: lock still active", lp);
244 }
245
246 lp->rw_wwwh = RW_DOUBLE_LOCK;
247 }
248
249 /*
250 * Verify that an rwlock is held correctly.
251 */
252 static int
rw_locked(rwlock_impl_t * lp,krw_t rw)253 rw_locked(rwlock_impl_t *lp, krw_t rw)
254 {
255 uintptr_t old = lp->rw_wwwh;
256
257 if (rw == RW_READER || rw == RW_READER_STARVEWRITER)
258 return ((old & RW_LOCKED) && !(old & RW_WRITE_LOCKED));
259
260 if (rw == RW_WRITER)
261 return ((old & RW_OWNER) == (uintptr_t)curthread);
262
263 return (0);
264 }
265
266 uint_t (*rw_lock_backoff)(uint_t) = NULL;
267 void (*rw_lock_delay)(uint_t) = NULL;
268
269 /*
270 * Full-service implementation of rw_enter() to handle all the hard cases.
271 * Called from the assembly version if anything complicated is going on.
272 * The only semantic difference between calling rw_enter() and calling
273 * rw_enter_sleep() directly is that we assume the caller has already done
274 * a THREAD_KPRI_REQUEST() in the RW_READER cases.
275 */
276 void
rw_enter_sleep(rwlock_impl_t * lp,krw_t rw)277 rw_enter_sleep(rwlock_impl_t *lp, krw_t rw)
278 {
279 uintptr_t old, new, lock_value, lock_busy, lock_wait;
280 hrtime_t sleep_time;
281 turnstile_t *ts;
282 uint_t backoff = 0;
283 int loop_count = 0;
284
285 if (rw == RW_READER) {
286 lock_value = RW_READ_LOCK;
287 lock_busy = RW_WRITE_CLAIMED;
288 lock_wait = RW_HAS_WAITERS;
289 } else if (rw == RW_READER_STARVEWRITER) {
290 lock_value = RW_READ_LOCK;
291 lock_busy = RW_WRITE_LOCKED;
292 lock_wait = RW_HAS_WAITERS;
293 } else {
294 lock_value = RW_WRITE_LOCK(curthread);
295 lock_busy = (uintptr_t)RW_LOCKED;
296 lock_wait = RW_HAS_WAITERS | RW_WRITE_WANTED;
297 }
298
299 for (;;) {
300 if (((old = lp->rw_wwwh) & lock_busy) == 0) {
301 if (casip(&lp->rw_wwwh, old, old + lock_value) != old) {
302 if (rw_lock_delay != NULL) {
303 backoff = rw_lock_backoff(backoff);
304 rw_lock_delay(backoff);
305 if (++loop_count == ncpus_online) {
306 backoff = 0;
307 loop_count = 0;
308 }
309 }
310 continue;
311 }
312 break;
313 }
314
315 if (panicstr)
316 return;
317
318 if ((old & RW_DOUBLE_LOCK) == RW_DOUBLE_LOCK) {
319 rw_panic("rw_enter: bad rwlock", lp);
320 return;
321 }
322
323 if ((old & RW_OWNER) == (uintptr_t)curthread) {
324 rw_panic("recursive rw_enter", lp);
325 return;
326 }
327
328 ts = turnstile_lookup(lp);
329
330 do {
331 if (((old = lp->rw_wwwh) & lock_busy) == 0)
332 break;
333 new = old | lock_wait;
334 } while (old != new && casip(&lp->rw_wwwh, old, new) != old);
335
336 if ((old & lock_busy) == 0) {
337 /*
338 * The lock appears free now; try the dance again
339 */
340 turnstile_exit(lp);
341 continue;
342 }
343
344 /*
345 * We really are going to block. Bump the stats, and drop
346 * kpri if we're a reader.
347 */
348 ASSERT(lp->rw_wwwh & lock_wait);
349 ASSERT(lp->rw_wwwh & RW_LOCKED);
350
351 sleep_time = -gethrtime();
352 if (rw != RW_WRITER) {
353 THREAD_KPRI_RELEASE();
354 CPU_STATS_ADDQ(CPU, sys, rw_rdfails, 1);
355 (void) turnstile_block(ts, TS_READER_Q, lp,
356 &rw_sobj_ops, NULL, NULL);
357 } else {
358 CPU_STATS_ADDQ(CPU, sys, rw_wrfails, 1);
359 (void) turnstile_block(ts, TS_WRITER_Q, lp,
360 &rw_sobj_ops, NULL, NULL);
361 }
362 sleep_time += gethrtime();
363
364 LOCKSTAT_RECORD4(LS_RW_ENTER_BLOCK, lp, sleep_time, rw,
365 (old & RW_WRITE_LOCKED) ? 1 : 0,
366 old >> RW_HOLD_COUNT_SHIFT);
367
368 /*
369 * We wake up holding the lock (and having kpri if we're
370 * a reader) via direct handoff from the previous owner.
371 */
372 break;
373 }
374
375 ASSERT(rw_locked(lp, rw));
376
377 membar_enter();
378
379 LOCKSTAT_RECORD(LS_RW_ENTER_ACQUIRE, lp, rw);
380 }
381
382 /*
383 * Return the number of readers to wake, or zero if we should wake a writer.
384 * Called only by exiting/downgrading writers (readers don't wake readers).
385 */
386 static int
rw_readers_to_wake(turnstile_t * ts)387 rw_readers_to_wake(turnstile_t *ts)
388 {
389 kthread_t *next_writer = ts->ts_sleepq[TS_WRITER_Q].sq_first;
390 kthread_t *next_reader = ts->ts_sleepq[TS_READER_Q].sq_first;
391 pri_t wpri = (next_writer != NULL) ? DISP_PRIO(next_writer) : -1;
392 int count = 0;
393
394 while (next_reader != NULL) {
395 if (DISP_PRIO(next_reader) < wpri)
396 break;
397 next_reader->t_kpri_req++;
398 next_reader = next_reader->t_link;
399 count++;
400 }
401 return (count);
402 }
403
404 /*
405 * Full-service implementation of rw_exit() to handle all the hard cases.
406 * Called from the assembly version if anything complicated is going on.
407 * There is no semantic difference between calling rw_exit() and calling
408 * rw_exit_wakeup() directly.
409 */
410 void
rw_exit_wakeup(rwlock_impl_t * lp)411 rw_exit_wakeup(rwlock_impl_t *lp)
412 {
413 turnstile_t *ts;
414 uintptr_t old, new, lock_value;
415 kthread_t *next_writer;
416 int nreaders;
417 uint_t backoff = 0;
418 int loop_count = 0;
419
420 membar_exit();
421
422 old = lp->rw_wwwh;
423 if (old & RW_WRITE_LOCKED) {
424 if ((old & RW_OWNER) != (uintptr_t)curthread) {
425 rw_panic("rw_exit: not owner", lp);
426 lp->rw_wwwh = 0;
427 return;
428 }
429 lock_value = RW_WRITE_LOCK(curthread);
430 } else {
431 if ((old & RW_LOCKED) == 0) {
432 rw_panic("rw_exit: lock not held", lp);
433 return;
434 }
435 lock_value = RW_READ_LOCK;
436 }
437
438 for (;;) {
439 /*
440 * If this is *not* the final exit of a lock with waiters,
441 * just drop the lock -- there's nothing tricky going on.
442 */
443 old = lp->rw_wwwh;
444 new = old - lock_value;
445 if ((new & (RW_LOCKED | RW_HAS_WAITERS)) != RW_HAS_WAITERS) {
446 if (casip(&lp->rw_wwwh, old, new) != old) {
447 if (rw_lock_delay != NULL) {
448 backoff = rw_lock_backoff(backoff);
449 rw_lock_delay(backoff);
450 if (++loop_count == ncpus_online) {
451 backoff = 0;
452 loop_count = 0;
453 }
454 }
455 continue;
456 }
457 break;
458 }
459
460 /*
461 * This appears to be the final exit of a lock with waiters.
462 * If we do not have the lock as writer (that is, if this is
463 * the last exit of a reader with waiting writers), we will
464 * grab the lock as writer to prevent additional readers.
465 * (This is required because a reader that is acquiring the
466 * lock via RW_READER_STARVEWRITER will not observe the
467 * RW_WRITE_WANTED bit -- and we could therefore be racing
468 * with such readers here.)
469 */
470 if (!(old & RW_WRITE_LOCKED)) {
471 new = RW_WRITE_LOCK(curthread) |
472 RW_HAS_WAITERS | RW_WRITE_WANTED;
473
474 if (casip(&lp->rw_wwwh, old, new) != old)
475 continue;
476 }
477
478 /*
479 * Perform the final exit of a lock that has waiters.
480 */
481 ts = turnstile_lookup(lp);
482
483 next_writer = ts->ts_sleepq[TS_WRITER_Q].sq_first;
484
485 if ((old & RW_WRITE_LOCKED) &&
486 (nreaders = rw_readers_to_wake(ts)) > 0) {
487 /*
488 * Don't drop the lock -- just set the hold count
489 * such that we grant the lock to all readers at once.
490 */
491 new = nreaders * RW_READ_LOCK;
492 if (ts->ts_waiters > nreaders)
493 new |= RW_HAS_WAITERS;
494 if (next_writer)
495 new |= RW_WRITE_WANTED;
496 lp->rw_wwwh = new;
497 membar_enter();
498 turnstile_wakeup(ts, TS_READER_Q, nreaders, NULL);
499 } else {
500 /*
501 * Don't drop the lock -- just transfer ownership
502 * directly to next_writer. Note that there must
503 * be at least one waiting writer, because we get
504 * here only if (A) the lock is read-locked or
505 * (B) there are no waiting readers. In case (A),
506 * since the lock is read-locked there would be no
507 * reason for other readers to have blocked unless
508 * the RW_WRITE_WANTED bit was set. In case (B),
509 * since there are waiters but no waiting readers,
510 * they must all be waiting writers.
511 */
512 ASSERT(lp->rw_wwwh & RW_WRITE_WANTED);
513 new = RW_WRITE_LOCK(next_writer);
514 if (ts->ts_waiters > 1)
515 new |= RW_HAS_WAITERS;
516 if (next_writer->t_link)
517 new |= RW_WRITE_WANTED;
518 lp->rw_wwwh = new;
519 membar_enter();
520 turnstile_wakeup(ts, TS_WRITER_Q, 1, next_writer);
521 }
522 break;
523 }
524
525 if (lock_value == RW_READ_LOCK) {
526 THREAD_KPRI_RELEASE();
527 LOCKSTAT_RECORD(LS_RW_EXIT_RELEASE, lp, RW_READER);
528 } else {
529 LOCKSTAT_RECORD(LS_RW_EXIT_RELEASE, lp, RW_WRITER);
530 }
531 }
532
533 int
rw_tryenter(krwlock_t * rwlp,krw_t rw)534 rw_tryenter(krwlock_t *rwlp, krw_t rw)
535 {
536 rwlock_impl_t *lp = (rwlock_impl_t *)rwlp;
537 uintptr_t old;
538
539 if (rw != RW_WRITER) {
540 uint_t backoff = 0;
541 int loop_count = 0;
542 THREAD_KPRI_REQUEST();
543 for (;;) {
544 if ((old = lp->rw_wwwh) & (rw == RW_READER ?
545 RW_WRITE_CLAIMED : RW_WRITE_LOCKED)) {
546 THREAD_KPRI_RELEASE();
547 return (0);
548 }
549 if (casip(&lp->rw_wwwh, old, old + RW_READ_LOCK) == old)
550 break;
551 if (rw_lock_delay != NULL) {
552 backoff = rw_lock_backoff(backoff);
553 rw_lock_delay(backoff);
554 if (++loop_count == ncpus_online) {
555 backoff = 0;
556 loop_count = 0;
557 }
558 }
559 }
560 LOCKSTAT_RECORD(LS_RW_TRYENTER_ACQUIRE, lp, rw);
561 } else {
562 if (casip(&lp->rw_wwwh, 0, RW_WRITE_LOCK(curthread)) != 0)
563 return (0);
564 LOCKSTAT_RECORD(LS_RW_TRYENTER_ACQUIRE, lp, rw);
565 }
566 ASSERT(rw_locked(lp, rw));
567 membar_enter();
568 return (1);
569 }
570
571 void
rw_downgrade(krwlock_t * rwlp)572 rw_downgrade(krwlock_t *rwlp)
573 {
574 rwlock_impl_t *lp = (rwlock_impl_t *)rwlp;
575
576 THREAD_KPRI_REQUEST();
577 membar_exit();
578
579 if ((lp->rw_wwwh & RW_OWNER) != (uintptr_t)curthread) {
580 rw_panic("rw_downgrade: not owner", lp);
581 return;
582 }
583
584 if (atomic_add_ip_nv(&lp->rw_wwwh,
585 RW_READ_LOCK - RW_WRITE_LOCK(curthread)) & RW_HAS_WAITERS) {
586 turnstile_t *ts = turnstile_lookup(lp);
587 int nreaders = rw_readers_to_wake(ts);
588 if (nreaders > 0) {
589 uintptr_t delta = nreaders * RW_READ_LOCK;
590 if (ts->ts_waiters == nreaders)
591 delta -= RW_HAS_WAITERS;
592 atomic_add_ip(&lp->rw_wwwh, delta);
593 }
594 turnstile_wakeup(ts, TS_READER_Q, nreaders, NULL);
595 }
596 ASSERT(rw_locked(lp, RW_READER));
597 LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, lp);
598 }
599
600 int
rw_tryupgrade(krwlock_t * rwlp)601 rw_tryupgrade(krwlock_t *rwlp)
602 {
603 rwlock_impl_t *lp = (rwlock_impl_t *)rwlp;
604 uintptr_t old, new;
605
606 ASSERT(rw_locked(lp, RW_READER));
607
608 do {
609 if (((old = lp->rw_wwwh) & ~RW_HAS_WAITERS) != RW_READ_LOCK)
610 return (0);
611 new = old + RW_WRITE_LOCK(curthread) - RW_READ_LOCK;
612 } while (casip(&lp->rw_wwwh, old, new) != old);
613
614 membar_enter();
615 THREAD_KPRI_RELEASE();
616 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, lp);
617 ASSERT(rw_locked(lp, RW_WRITER));
618 return (1);
619 }
620
621 int
rw_read_held(krwlock_t * rwlp)622 rw_read_held(krwlock_t *rwlp)
623 {
624 uintptr_t tmp;
625
626 return (_RW_READ_HELD(rwlp, tmp));
627 }
628
629 int
rw_write_held(krwlock_t * rwlp)630 rw_write_held(krwlock_t *rwlp)
631 {
632 return (_RW_WRITE_HELD(rwlp));
633 }
634
635 int
rw_lock_held(krwlock_t * rwlp)636 rw_lock_held(krwlock_t *rwlp)
637 {
638 return (_RW_LOCK_HELD(rwlp));
639 }
640
641 /*
642 * Like rw_read_held(), but ASSERTs that the lock is currently held
643 */
644 int
rw_read_locked(krwlock_t * rwlp)645 rw_read_locked(krwlock_t *rwlp)
646 {
647 uintptr_t old = ((rwlock_impl_t *)rwlp)->rw_wwwh;
648
649 ASSERT(old & RW_LOCKED);
650 return ((old & RW_LOCKED) && !(old & RW_WRITE_LOCKED));
651 }
652
653 /*
654 * Returns non-zero if the lock is either held or desired by a writer
655 */
656 int
rw_iswriter(krwlock_t * rwlp)657 rw_iswriter(krwlock_t *rwlp)
658 {
659 return (_RW_ISWRITER(rwlp));
660 }
661
662 kthread_t *
rw_owner(krwlock_t * rwlp)663 rw_owner(krwlock_t *rwlp)
664 {
665 uintptr_t old = ((rwlock_impl_t *)rwlp)->rw_wwwh;
666
667 return ((old & RW_WRITE_LOCKED) ? (kthread_t *)(old & RW_OWNER) : NULL);
668 }
669