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
rwbase_read_trylock(struct rwbase_rt * rwb)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 /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
63 if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
64 return 1;
65 }
66 return 0;
67 }
68
__rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)69 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
70 unsigned int state)
71 {
72 struct rt_mutex_base *rtm = &rwb->rtmutex;
73 int ret;
74
75 raw_spin_lock_irq(&rtm->wait_lock);
76 /*
77 * Allow readers, as long as the writer has not completely
78 * acquired the semaphore for write.
79 */
80 if (atomic_read(&rwb->readers) != WRITER_BIAS) {
81 atomic_inc(&rwb->readers);
82 raw_spin_unlock_irq(&rtm->wait_lock);
83 return 0;
84 }
85
86 /*
87 * Call into the slow lock path with the rtmutex->wait_lock
88 * held, so this can't result in the following race:
89 *
90 * Reader1 Reader2 Writer
91 * down_read()
92 * down_write()
93 * rtmutex_lock(m)
94 * wait()
95 * down_read()
96 * unlock(m->wait_lock)
97 * up_read()
98 * wake(Writer)
99 * lock(m->wait_lock)
100 * sem->writelocked=true
101 * unlock(m->wait_lock)
102 *
103 * up_write()
104 * sem->writelocked=false
105 * rtmutex_unlock(m)
106 * down_read()
107 * down_write()
108 * rtmutex_lock(m)
109 * wait()
110 * rtmutex_lock(m)
111 *
112 * That would put Reader1 behind the writer waiting on
113 * Reader2 to call up_read(), which might be unbound.
114 */
115
116 /*
117 * For rwlocks this returns 0 unconditionally, so the below
118 * !ret conditionals are optimized out.
119 */
120 ret = rwbase_rtmutex_slowlock_locked(rtm, state);
121
122 /*
123 * On success the rtmutex is held, so there can't be a writer
124 * active. Increment the reader count and immediately drop the
125 * rtmutex again.
126 *
127 * rtmutex->wait_lock has to be unlocked in any case of course.
128 */
129 if (!ret)
130 atomic_inc(&rwb->readers);
131 raw_spin_unlock_irq(&rtm->wait_lock);
132 if (!ret)
133 rwbase_rtmutex_unlock(rtm);
134 return ret;
135 }
136
rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)137 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
138 unsigned int state)
139 {
140 if (rwbase_read_trylock(rwb))
141 return 0;
142
143 return __rwbase_read_lock(rwb, state);
144 }
145
__rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)146 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
147 unsigned int state)
148 {
149 struct rt_mutex_base *rtm = &rwb->rtmutex;
150 struct task_struct *owner;
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 wake_up_state(owner, state);
162
163 raw_spin_unlock_irq(&rtm->wait_lock);
164 }
165
rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)166 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
167 unsigned int state)
168 {
169 /*
170 * rwb->readers can only hit 0 when a writer is waiting for the
171 * active readers to leave the critical section.
172 *
173 * dec_and_test() is fully ordered, provides RELEASE.
174 */
175 if (unlikely(atomic_dec_and_test(&rwb->readers)))
176 __rwbase_read_unlock(rwb, state);
177 }
178
__rwbase_write_unlock(struct rwbase_rt * rwb,int bias,unsigned long flags)179 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
180 unsigned long flags)
181 {
182 struct rt_mutex_base *rtm = &rwb->rtmutex;
183
184 /*
185 * _release() is needed in case that reader is in fast path, pairing
186 * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
187 */
188 (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
189 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
190 rwbase_rtmutex_unlock(rtm);
191 }
192
rwbase_write_unlock(struct rwbase_rt * rwb)193 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
194 {
195 struct rt_mutex_base *rtm = &rwb->rtmutex;
196 unsigned long flags;
197
198 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
199 __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
200 }
201
rwbase_write_downgrade(struct rwbase_rt * rwb)202 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
203 {
204 struct rt_mutex_base *rtm = &rwb->rtmutex;
205 unsigned long flags;
206
207 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
208 /* Release it and account current as reader */
209 __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
210 }
211
__rwbase_write_trylock(struct rwbase_rt * rwb)212 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
213 {
214 /* Can do without CAS because we're serialized by wait_lock. */
215 lockdep_assert_held(&rwb->rtmutex.wait_lock);
216
217 /*
218 * _acquire is needed in case the reader is in the fast path, pairing
219 * with rwbase_read_unlock(), provides ACQUIRE.
220 */
221 if (!atomic_read_acquire(&rwb->readers)) {
222 atomic_set(&rwb->readers, WRITER_BIAS);
223 return 1;
224 }
225
226 return 0;
227 }
228
rwbase_write_lock(struct rwbase_rt * rwb,unsigned int state)229 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
230 unsigned int state)
231 {
232 struct rt_mutex_base *rtm = &rwb->rtmutex;
233 unsigned long flags;
234
235 /* Take the rtmutex as a first step */
236 if (rwbase_rtmutex_lock_state(rtm, state))
237 return -EINTR;
238
239 /* Force readers into slow path */
240 atomic_sub(READER_BIAS, &rwb->readers);
241
242 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
243 if (__rwbase_write_trylock(rwb))
244 goto out_unlock;
245
246 rwbase_set_and_save_current_state(state);
247 for (;;) {
248 /* Optimized out for rwlocks */
249 if (rwbase_signal_pending_state(state, current)) {
250 rwbase_restore_current_state();
251 __rwbase_write_unlock(rwb, 0, flags);
252 return -EINTR;
253 }
254
255 if (__rwbase_write_trylock(rwb))
256 break;
257
258 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
259 rwbase_schedule();
260 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
261
262 set_current_state(state);
263 }
264 rwbase_restore_current_state();
265
266 out_unlock:
267 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
268 return 0;
269 }
270
rwbase_write_trylock(struct rwbase_rt * rwb)271 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
272 {
273 struct rt_mutex_base *rtm = &rwb->rtmutex;
274 unsigned long flags;
275
276 if (!rwbase_rtmutex_trylock(rtm))
277 return 0;
278
279 atomic_sub(READER_BIAS, &rwb->readers);
280
281 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
282 if (__rwbase_write_trylock(rwb)) {
283 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
284 return 1;
285 }
286 __rwbase_write_unlock(rwb, 0, flags);
287 return 0;
288 }
289