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
53static __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
68static 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
139static __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
150static 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
174static __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
187static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
188 unsigned long flags)
189{
190 struct rt_mutex_base *rtm = &rwb->rtmutex;
191
192 /*
193 * _release() is needed in case that reader is in fast path, pairing
194 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
195 */
196 (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
197 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
198 rwbase_rtmutex_unlock(rtm);
199}
200
201static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
202{
203 struct rt_mutex_base *rtm = &rwb->rtmutex;
204 unsigned long flags;
205
206 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
207 __rwbase_write_unlock(rwb, bias: WRITER_BIAS, flags);
208}
209
210static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
211{
212 struct rt_mutex_base *rtm = &rwb->rtmutex;
213 unsigned long flags;
214
215 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
216 /* Release it and account current as reader */
217 __rwbase_write_unlock(rwb, bias: WRITER_BIAS - 1, flags);
218}
219
220static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
221{
222 /* Can do without CAS because we're serialized by wait_lock. */
223 lockdep_assert_held(&rwb->rtmutex.wait_lock);
224
225 /*
226 * _acquire is needed in case the reader is in the fast path, pairing
227 * with rwbase_read_unlock(), provides ACQUIRE.
228 */
229 if (!atomic_read_acquire(&rwb->readers)) {
230 atomic_set(&rwb->readers, WRITER_BIAS);
231 return 1;
232 }
233
234 return 0;
235}
236
237static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
238 unsigned int state)
239{
240 struct rt_mutex_base *rtm = &rwb->rtmutex;
241 unsigned long flags;
242
243 /* Take the rtmutex as a first step */
244 if (rwbase_rtmutex_lock_state(rtm, state))
245 return -EINTR;
246
247 /* Force readers into slow path */
248 atomic_sub(READER_BIAS, &rwb->readers);
249
250 rwbase_pre_schedule();
251
252 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
253 if (__rwbase_write_trylock(rwb))
254 goto out_unlock;
255
256 rwbase_set_and_save_current_state(state);
257 trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
258 for (;;) {
259 /* Optimized out for rwlocks */
260 if (rwbase_signal_pending_state(state, current)) {
261 rwbase_restore_current_state();
262 __rwbase_write_unlock(rwb, 0, flags);
263 rwbase_post_schedule();
264 trace_contention_end(rwb, -EINTR);
265 return -EINTR;
266 }
267
268 if (__rwbase_write_trylock(rwb))
269 break;
270
271 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
272 rwbase_schedule();
273 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
274
275 set_current_state(state);
276 }
277 rwbase_restore_current_state();
278 trace_contention_end(rwb, 0);
279
280out_unlock:
281 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
282 rwbase_post_schedule();
283 return 0;
284}
285
286static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
287{
288 struct rt_mutex_base *rtm = &rwb->rtmutex;
289 unsigned long flags;
290
291 if (!rwbase_rtmutex_trylock(rtm))
292 return 0;
293
294 atomic_sub(READER_BIAS, &rwb->readers);
295
296 raw_spin_lock_irqsave(&rtm->wait_lock, flags);
297 if (__rwbase_write_trylock(rwb)) {
298 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
299 return 1;
300 }
301 __rwbase_write_unlock(rwb, bias: 0, flags);
302 return 0;
303}
304

source code of linux/kernel/locking/rwbase_rt.c