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 	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 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
116 
117 	/*
118 	 * For rwlocks this returns 0 unconditionally, so the below
119 	 * !ret conditionals are optimized out.
120 	 */
121 	ret = rwbase_rtmutex_slowlock_locked(rtm, state);
122 
123 	/*
124 	 * On success the rtmutex is held, so there can't be a writer
125 	 * active. Increment the reader count and immediately drop the
126 	 * rtmutex again.
127 	 *
128 	 * rtmutex->wait_lock has to be unlocked in any case of course.
129 	 */
130 	if (!ret)
131 		atomic_inc(&rwb->readers);
132 	raw_spin_unlock_irq(&rtm->wait_lock);
133 	if (!ret)
134 		rwbase_rtmutex_unlock(rtm);
135 
136 	trace_contention_end(rwb, ret);
137 	return ret;
138 }
139 
rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)140 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
141 					    unsigned int state)
142 {
143 	if (rwbase_read_trylock(rwb))
144 		return 0;
145 
146 	return __rwbase_read_lock(rwb, state);
147 }
148 
__rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)149 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
150 					 unsigned int state)
151 {
152 	struct rt_mutex_base *rtm = &rwb->rtmutex;
153 	struct task_struct *owner;
154 	DEFINE_RT_WAKE_Q(wqh);
155 
156 	raw_spin_lock_irq(&rtm->wait_lock);
157 	/*
158 	 * Wake the writer, i.e. the rtmutex owner. It might release the
159 	 * rtmutex concurrently in the fast path (due to a signal), but to
160 	 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
161 	 * worst case which can happen is a spurious wakeup.
162 	 */
163 	owner = rt_mutex_owner(rtm);
164 	if (owner)
165 		rt_mutex_wake_q_add_task(&wqh, owner, state);
166 
167 	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
168 	preempt_disable();
169 	raw_spin_unlock_irq(&rtm->wait_lock);
170 	rt_mutex_wake_up_q(&wqh);
171 }
172 
rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)173 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
174 					       unsigned int state)
175 {
176 	/*
177 	 * rwb->readers can only hit 0 when a writer is waiting for the
178 	 * active readers to leave the critical section.
179 	 *
180 	 * dec_and_test() is fully ordered, provides RELEASE.
181 	 */
182 	if (unlikely(atomic_dec_and_test(&rwb->readers)))
183 		__rwbase_read_unlock(rwb, state);
184 }
185 
__rwbase_write_unlock(struct rwbase_rt * rwb,int bias,unsigned long flags)186 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
187 					 unsigned long flags)
188 {
189 	struct rt_mutex_base *rtm = &rwb->rtmutex;
190 
191 	/*
192 	 * _release() is needed in case that reader is in fast path, pairing
193 	 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
194 	 */
195 	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
196 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
197 	rwbase_rtmutex_unlock(rtm);
198 }
199 
rwbase_write_unlock(struct rwbase_rt * rwb)200 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
201 {
202 	struct rt_mutex_base *rtm = &rwb->rtmutex;
203 	unsigned long flags;
204 
205 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
206 	__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
207 }
208 
rwbase_write_downgrade(struct rwbase_rt * rwb)209 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
210 {
211 	struct rt_mutex_base *rtm = &rwb->rtmutex;
212 	unsigned long flags;
213 
214 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
215 	/* Release it and account current as reader */
216 	__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
217 }
218 
__rwbase_write_trylock(struct rwbase_rt * rwb)219 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
220 {
221 	/* Can do without CAS because we're serialized by wait_lock. */
222 	lockdep_assert_held(&rwb->rtmutex.wait_lock);
223 
224 	/*
225 	 * _acquire is needed in case the reader is in the fast path, pairing
226 	 * with rwbase_read_unlock(), provides ACQUIRE.
227 	 */
228 	if (!atomic_read_acquire(&rwb->readers)) {
229 		atomic_set(&rwb->readers, WRITER_BIAS);
230 		return 1;
231 	}
232 
233 	return 0;
234 }
235 
rwbase_write_lock(struct rwbase_rt * rwb,unsigned int state)236 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
237 				     unsigned int state)
238 {
239 	struct rt_mutex_base *rtm = &rwb->rtmutex;
240 	unsigned long flags;
241 
242 	/* Take the rtmutex as a first step */
243 	if (rwbase_rtmutex_lock_state(rtm, state))
244 		return -EINTR;
245 
246 	/* Force readers into slow path */
247 	atomic_sub(READER_BIAS, &rwb->readers);
248 
249 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
250 	if (__rwbase_write_trylock(rwb))
251 		goto out_unlock;
252 
253 	rwbase_set_and_save_current_state(state);
254 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
255 	for (;;) {
256 		/* Optimized out for rwlocks */
257 		if (rwbase_signal_pending_state(state, current)) {
258 			rwbase_restore_current_state();
259 			__rwbase_write_unlock(rwb, 0, flags);
260 			trace_contention_end(rwb, -EINTR);
261 			return -EINTR;
262 		}
263 
264 		if (__rwbase_write_trylock(rwb))
265 			break;
266 
267 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
268 		rwbase_schedule();
269 		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
270 
271 		set_current_state(state);
272 	}
273 	rwbase_restore_current_state();
274 	trace_contention_end(rwb, 0);
275 
276 out_unlock:
277 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
278 	return 0;
279 }
280 
rwbase_write_trylock(struct rwbase_rt * rwb)281 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
282 {
283 	struct rt_mutex_base *rtm = &rwb->rtmutex;
284 	unsigned long flags;
285 
286 	if (!rwbase_rtmutex_trylock(rtm))
287 		return 0;
288 
289 	atomic_sub(READER_BIAS, &rwb->readers);
290 
291 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
292 	if (__rwbase_write_trylock(rwb)) {
293 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
294 		return 1;
295 	}
296 	__rwbase_write_unlock(rwb, 0, flags);
297 	return 0;
298 }
299