1 // SPDX-License-Identifier: GPL-2.0-only
2
3 /*
4 * RT-specific reader/writer semaphores and reader/writer locks
5 *
6 * down_write/write_lock()
7 * 1) Lock rtmutex
8 * 2) Remove the reader BIAS to force readers into the slow path
9 * 3) Wait until all readers have left the critical section
10 * 4) Mark it write locked
11 *
12 * up_write/write_unlock()
13 * 1) Remove the write locked marker
14 * 2) Set the reader BIAS, so readers can use the fast path again
15 * 3) Unlock rtmutex, to release blocked readers
16 *
17 * down_read/read_lock()
18 * 1) Try fast path acquisition (reader BIAS is set)
19 * 2) Take tmutex::wait_lock, which protects the writelocked flag
20 * 3) If !writelocked, acquire it for read
21 * 4) If writelocked, block on tmutex
22 * 5) unlock rtmutex, goto 1)
23 *
24 * up_read/read_unlock()
25 * 1) Try fast path release (reader count != 1)
26 * 2) Wake the writer waiting in down_write()/write_lock() #3
27 *
28 * down_read/read_lock()#3 has the consequence, that rw semaphores and rw
29 * locks on RT are not writer fair, but writers, which should be avoided in
30 * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
31 * inheritance mechanism.
32 *
33 * It's possible to make the rw primitives writer fair by keeping a list of
34 * active readers. A blocked writer would force all newly incoming readers
35 * to block on the rtmutex, but the rtmutex would have to be proxy locked
36 * for one reader after the other. We can't use multi-reader inheritance
37 * because there is no way to support that with SCHED_DEADLINE.
38 * Implementing the one by one reader boosting/handover mechanism is a
39 * major surgery for a very dubious value.
40 *
41 * The risk of writer starvation is there, but the pathological use cases
42 * which trigger it are not necessarily the typical RT workloads.
43 *
44 * Fast-path orderings:
45 * The lock/unlock of readers can run in fast paths: lock and unlock are only
46 * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
47 * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
48 * and _release() (or stronger).
49 *
50 * Common code shared between RT rw_semaphore and rwlock
51 */
52
53 static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
54 {
55 int r;
56
57 /*
58 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
59 * set.
60 */
61 for (r = atomic_read(&rwb->readers); r < 0;) {
62 if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))
63 return 1;
64 }
65 return 0;
66 }
67
68 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
69 unsigned int state)
70 {
71 struct rt_mutex_base *rtm = &rwb->rtmutex;
72 int ret;
73
74 raw_spin_lock_irq(&rtm->wait_lock);
75 /*
76 * Allow readers, as long as the writer has not completely
77 * acquired the semaphore for write.
78 */
79 if (atomic_read(&rwb->readers) != WRITER_BIAS) {
80 atomic_inc(&rwb->readers);
81 raw_spin_unlock_irq(&rtm->wait_lock);
82 return 0;
83 }
84
85 /*
86 * Call into the slow lock path with the rtmutex->wait_lock
87 * held, so this can't result in the following race:
88 *
89 * Reader1 Reader2 Writer
90 * down_read()
91 * down_write()
92 * rtmutex_lock(m)
93 * wait()
94 * down_read()
95 * unlock(m->wait_lock)
96 * up_read()
97 * wake(Writer)
98 * lock(m->wait_lock)
99 * sem->writelocked=true
100 * unlock(m->wait_lock)
101 *
102 * up_write()
103 * sem->writelocked=false
104 * rtmutex_unlock(m)
105 * down_read()
106 * down_write()
107 * rtmutex_lock(m)
108 * wait()
109 * rtmutex_lock(m)
110 *
111 * That would put Reader1 behind the writer waiting on
112 * Reader2 to call up_read(), which might be unbound.
113 */
114
115 /*
116 * For rwlocks this returns 0 unconditionally, so the below
117 * !ret conditionals are optimized out.
118 */
119 ret = rwbase_rtmutex_slowlock_locked(rtm, state);
120
121 /*
122 * On success the rtmutex is held, so there can't be a writer
123 * active. Increment the reader count and immediately drop the
124 * rtmutex again.
125 *
126 * rtmutex->wait_lock has to be unlocked in any case of course.
127 */
128 if (!ret)
129 atomic_inc(&rwb->readers);
130 raw_spin_unlock_irq(&rtm->wait_lock);
131 if (!ret)
132 rwbase_rtmutex_unlock(rtm);
133 return ret;
134 }
135
136 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
137 unsigned int state)
138 {
139 if (rwbase_read_trylock(rwb))
140 return 0;
141
142 return __rwbase_read_lock(rwb, state);
143 }
144
145 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
146 unsigned int state)
147 {
148 struct rt_mutex_base *rtm = &rwb->rtmutex;
149 struct task_struct *owner;
150 DEFINE_RT_WAKE_Q(wqh);
151
152 raw_spin_lock_irq(&rtm->wait_lock);
153 /*
154 * Wake the writer, i.e. the rtmutex owner. It might release the
155 * rtmutex concurrently in the fast path (due to a signal), but to
156 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
157 * worst case which can happen is a spurious wakeup.
158 */
159 owner = rt_mutex_owner(rtm);
160 if (owner)
161 rt_mutex_wake_q_add_task(&wqh, owner, state);
162
163 /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
164 preempt_disable();
165 raw_spin_unlock_irq(&rtm->wait_lock);
166 rt_mutex_wake_up_q(&wqh);
167 }
168
169 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
170 unsigned int state)
171 {
172 /*
173 * rwb->readers can only hit 0 when a writer is waiting for the
174 * active readers to leave the critical section.
175 *
176 * dec_and_test() is fully ordered, provides RELEASE.
177 */
178 if (unlikely(atomic_dec_and_test(&rwb->readers)))
179 __rwbase_read_unlock(rwb, state);
180 }
181
182 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
183 unsigned long flags)
184 {
185 struct rt_mutex_base *rtm = &rwb->rtmutex;
186
187 /*
188 * _release() is needed in case that reader is in fast path, pairing
189 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
190 */
191 (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
192 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
193 rwbase_rtmutex_unlock(rtm);
194 }
195
196 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
197 {
198 struct rt_mutex_base *rtm = &rwb->rtmutex;
199 unsigned long flags;
200
201 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
202 __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
203 }
204
205 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
206 {
207 struct rt_mutex_base *rtm = &rwb->rtmutex;
208 unsigned long flags;
209
210 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
211 /* Release it and account current as reader */
212 __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
213 }
214
215 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
216 {
217 /* Can do without CAS because we're serialized by wait_lock. */
218 lockdep_assert_held(&rwb->rtmutex.wait_lock);
219
220 /*
221 * _acquire is needed in case the reader is in the fast path, pairing
222 * with rwbase_read_unlock(), provides ACQUIRE.
223 */
224 if (!atomic_read_acquire(&rwb->readers)) {
225 atomic_set(&rwb->readers, WRITER_BIAS);
226 return 1;
227 }
228
229 return 0;
230 }
231
232 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
233 unsigned int state)
234 {
235 struct rt_mutex_base *rtm = &rwb->rtmutex;
236 unsigned long flags;
237
238 /* Take the rtmutex as a first step */
239 if (rwbase_rtmutex_lock_state(rtm, state))
240 return -EINTR;
241
242 /* Force readers into slow path */
243 atomic_sub(READER_BIAS, &rwb->readers);
244
245 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
246 if (__rwbase_write_trylock(rwb))
247 goto out_unlock;
248
249 rwbase_set_and_save_current_state(state);
250 for (;;) {
251 /* Optimized out for rwlocks */
252 if (rwbase_signal_pending_state(state, current)) {
253 rwbase_restore_current_state();
254 __rwbase_write_unlock(rwb, 0, flags);
255 return -EINTR;
256 }
257
258 if (__rwbase_write_trylock(rwb))
259 break;
260
261 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
262 rwbase_schedule();
263 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
264
265 set_current_state(state);
266 }
267 rwbase_restore_current_state();
268
269 out_unlock:
270 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
271 return 0;
272 }
273
274 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
275 {
276 struct rt_mutex_base *rtm = &rwb->rtmutex;
277 unsigned long flags;
278
279 if (!rwbase_rtmutex_trylock(rtm))
280 return 0;
281
282 atomic_sub(READER_BIAS, &rwb->readers);
283
284 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
285 if (__rwbase_write_trylock(rwb)) {
286 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
287 return 1;
288 }
289 __rwbase_write_unlock(rwb, 0, flags);
290 return 0;
291 }
292
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