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 if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))
63 return 1;
64 }
65 return 0;
66 }
67
__rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)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 DEFINE_WAKE_Q(wake_q);
73 int ret;
74
75 rwbase_pre_schedule();
76 raw_spin_lock_irq(&rtm->wait_lock);
77
78 /*
79 * Call into the slow lock path with the rtmutex->wait_lock
80 * held, so this can't result in the following race:
81 *
82 * Reader1 Reader2 Writer
83 * down_read()
84 * down_write()
85 * rtmutex_lock(m)
86 * wait()
87 * down_read()
88 * unlock(m->wait_lock)
89 * up_read()
90 * wake(Writer)
91 * lock(m->wait_lock)
92 * sem->writelocked=true
93 * unlock(m->wait_lock)
94 *
95 * up_write()
96 * sem->writelocked=false
97 * rtmutex_unlock(m)
98 * down_read()
99 * down_write()
100 * rtmutex_lock(m)
101 * wait()
102 * rtmutex_lock(m)
103 *
104 * That would put Reader1 behind the writer waiting on
105 * Reader2 to call up_read(), which might be unbound.
106 */
107
108 trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
109
110 /*
111 * For rwlocks this returns 0 unconditionally, so the below
112 * !ret conditionals are optimized out.
113 */
114 ret = rwbase_rtmutex_slowlock_locked(rtm, state, &wake_q);
115
116 /*
117 * On success the rtmutex is held, so there can't be a writer
118 * active. Increment the reader count and immediately drop the
119 * rtmutex again.
120 *
121 * rtmutex->wait_lock has to be unlocked in any case of course.
122 */
123 if (!ret)
124 atomic_inc(&rwb->readers);
125
126 preempt_disable();
127 raw_spin_unlock_irq(&rtm->wait_lock);
128 wake_up_q(&wake_q);
129 preempt_enable();
130
131 if (!ret)
132 rwbase_rtmutex_unlock(rtm);
133
134 trace_contention_end(rwb, ret);
135 rwbase_post_schedule();
136 return ret;
137 }
138
rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)139 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
140 unsigned int state)
141 {
142 lockdep_assert(!current->pi_blocked_on);
143
144 if (rwbase_read_trylock(rwb))
145 return 0;
146
147 return __rwbase_read_lock(rwb, state);
148 }
149
__rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)150 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
151 unsigned int state)
152 {
153 struct rt_mutex_base *rtm = &rwb->rtmutex;
154 struct task_struct *owner;
155 DEFINE_RT_WAKE_Q(wqh);
156
157 raw_spin_lock_irq(&rtm->wait_lock);
158 /*
159 * Wake the writer, i.e. the rtmutex owner. It might release the
160 * rtmutex concurrently in the fast path (due to a signal), but to
161 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
162 * worst case which can happen is a spurious wakeup.
163 */
164 owner = rt_mutex_owner(rtm);
165 if (owner)
166 rt_mutex_wake_q_add_task(&wqh, owner, state);
167
168 /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
169 preempt_disable();
170 raw_spin_unlock_irq(&rtm->wait_lock);
171 rt_mutex_wake_up_q(&wqh);
172 }
173
rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)174 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
175 unsigned int state)
176 {
177 /*
178 * rwb->readers can only hit 0 when a writer is waiting for the
179 * active readers to leave the critical section.
180 *
181 * dec_and_test() is fully ordered, provides RELEASE.
182 */
183 if (unlikely(atomic_dec_and_test(&rwb->readers)))
184 __rwbase_read_unlock(rwb, state);
185 }
186
__rwbase_write_unlock(struct rwbase_rt * rwb,int bias,unsigned long flags)187 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
188 unsigned long flags)
189 __releases(&rwb->rtmutex.wait_lock)
190 {
191 struct rt_mutex_base *rtm = &rwb->rtmutex;
192
193 /*
194 * _release() is needed in case that reader is in fast path, pairing
195 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
196 */
197 (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
198 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
199 rwbase_rtmutex_unlock(rtm);
200 }
201
rwbase_write_unlock(struct rwbase_rt * rwb)202 static inline void rwbase_write_unlock(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 __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
209 }
210
rwbase_write_downgrade(struct rwbase_rt * rwb)211 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
212 {
213 struct rt_mutex_base *rtm = &rwb->rtmutex;
214 unsigned long flags;
215
216 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
217 /* Release it and account current as reader */
218 __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
219 }
220
__rwbase_write_trylock(struct rwbase_rt * rwb)221 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
222 {
223 /* Can do without CAS because we're serialized by wait_lock. */
224 lockdep_assert_held(&rwb->rtmutex.wait_lock);
225
226 /*
227 * _acquire is needed in case the reader is in the fast path, pairing
228 * with rwbase_read_unlock(), provides ACQUIRE.
229 */
230 if (!atomic_read_acquire(&rwb->readers)) {
231 atomic_set(&rwb->readers, WRITER_BIAS);
232 return 1;
233 }
234
235 return 0;
236 }
237
rwbase_write_lock(struct rwbase_rt * rwb,unsigned int state)238 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
239 unsigned int state)
240 {
241 struct rt_mutex_base *rtm = &rwb->rtmutex;
242 unsigned long flags;
243
244 /* Take the rtmutex as a first step */
245 if (rwbase_rtmutex_lock_state(rtm, state))
246 return -EINTR;
247
248 /* Force readers into slow path */
249 atomic_sub(READER_BIAS, &rwb->readers);
250
251 rwbase_pre_schedule();
252
253 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
254 if (__rwbase_write_trylock(rwb))
255 goto out_unlock;
256
257 rwbase_set_and_save_current_state(state);
258 trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
259 for (;;) {
260 /* Optimized out for rwlocks */
261 if (rwbase_signal_pending_state(state, current)) {
262 rwbase_restore_current_state();
263 __rwbase_write_unlock(rwb, 0, flags);
264 rwbase_post_schedule();
265 trace_contention_end(rwb, -EINTR);
266 return -EINTR;
267 }
268
269 if (__rwbase_write_trylock(rwb))
270 break;
271
272 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
273 rwbase_schedule();
274 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
275
276 set_current_state(state);
277 }
278 rwbase_restore_current_state();
279 trace_contention_end(rwb, 0);
280
281 out_unlock:
282 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
283 rwbase_post_schedule();
284 return 0;
285 }
286
rwbase_write_trylock(struct rwbase_rt * rwb)287 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
288 {
289 struct rt_mutex_base *rtm = &rwb->rtmutex;
290 unsigned long flags;
291
292 if (!rwbase_rtmutex_trylock(rtm))
293 return 0;
294
295 atomic_sub(READER_BIAS, &rwb->readers);
296
297 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
298 if (__rwbase_write_trylock(rwb)) {
299 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
300 return 1;
301 }
302 __rwbase_write_unlock(rwb, 0, flags);
303 return 0;
304 }
305