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 	/*
77 	 * Call into the slow lock path with the rtmutex->wait_lock
78 	 * held, so this can't result in the following race:
79 	 *
80 	 * Reader1		Reader2		Writer
81 	 *			down_read()
82 	 *					down_write()
83 	 *					rtmutex_lock(m)
84 	 *					wait()
85 	 * down_read()
86 	 * unlock(m->wait_lock)
87 	 *			up_read()
88 	 *			wake(Writer)
89 	 *					lock(m->wait_lock)
90 	 *					sem->writelocked=true
91 	 *					unlock(m->wait_lock)
92 	 *
93 	 *					up_write()
94 	 *					sem->writelocked=false
95 	 *					rtmutex_unlock(m)
96 	 *			down_read()
97 	 *					down_write()
98 	 *					rtmutex_lock(m)
99 	 *					wait()
100 	 * rtmutex_lock(m)
101 	 *
102 	 * That would put Reader1 behind the writer waiting on
103 	 * Reader2 to call up_read(), which might be unbound.
104 	 */
105 
106 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
107 
108 	/*
109 	 * For rwlocks this returns 0 unconditionally, so the below
110 	 * !ret conditionals are optimized out.
111 	 */
112 	ret = rwbase_rtmutex_slowlock_locked(rtm, state);
113 
114 	/*
115 	 * On success the rtmutex is held, so there can't be a writer
116 	 * active. Increment the reader count and immediately drop the
117 	 * rtmutex again.
118 	 *
119 	 * rtmutex->wait_lock has to be unlocked in any case of course.
120 	 */
121 	if (!ret)
122 		atomic_inc(&rwb->readers);
123 	raw_spin_unlock_irq(&rtm->wait_lock);
124 	if (!ret)
125 		rwbase_rtmutex_unlock(rtm);
126 
127 	trace_contention_end(rwb, ret);
128 	return ret;
129 }
130 
rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)131 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
132 					    unsigned int state)
133 {
134 	if (rwbase_read_trylock(rwb))
135 		return 0;
136 
137 	return __rwbase_read_lock(rwb, state);
138 }
139 
__rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)140 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
141 					 unsigned int state)
142 {
143 	struct rt_mutex_base *rtm = &rwb->rtmutex;
144 	struct task_struct *owner;
145 	DEFINE_RT_WAKE_Q(wqh);
146 
147 	raw_spin_lock_irq(&rtm->wait_lock);
148 	/*
149 	 * Wake the writer, i.e. the rtmutex owner. It might release the
150 	 * rtmutex concurrently in the fast path (due to a signal), but to
151 	 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
152 	 * worst case which can happen is a spurious wakeup.
153 	 */
154 	owner = rt_mutex_owner(rtm);
155 	if (owner)
156 		rt_mutex_wake_q_add_task(&wqh, owner, state);
157 
158 	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
159 	preempt_disable();
160 	raw_spin_unlock_irq(&rtm->wait_lock);
161 	rt_mutex_wake_up_q(&wqh);
162 }
163 
rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)164 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
165 					       unsigned int state)
166 {
167 	/*
168 	 * rwb->readers can only hit 0 when a writer is waiting for the
169 	 * active readers to leave the critical section.
170 	 *
171 	 * dec_and_test() is fully ordered, provides RELEASE.
172 	 */
173 	if (unlikely(atomic_dec_and_test(&rwb->readers)))
174 		__rwbase_read_unlock(rwb, state);
175 }
176 
__rwbase_write_unlock(struct rwbase_rt * rwb,int bias,unsigned long flags)177 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
178 					 unsigned long flags)
179 {
180 	struct rt_mutex_base *rtm = &rwb->rtmutex;
181 
182 	/*
183 	 * _release() is needed in case that reader is in fast path, pairing
184 	 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
185 	 */
186 	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
187 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
188 	rwbase_rtmutex_unlock(rtm);
189 }
190 
rwbase_write_unlock(struct rwbase_rt * rwb)191 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
192 {
193 	struct rt_mutex_base *rtm = &rwb->rtmutex;
194 	unsigned long flags;
195 
196 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
197 	__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
198 }
199 
rwbase_write_downgrade(struct rwbase_rt * rwb)200 static inline void rwbase_write_downgrade(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 	/* Release it and account current as reader */
207 	__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
208 }
209 
__rwbase_write_trylock(struct rwbase_rt * rwb)210 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
211 {
212 	/* Can do without CAS because we're serialized by wait_lock. */
213 	lockdep_assert_held(&rwb->rtmutex.wait_lock);
214 
215 	/*
216 	 * _acquire is needed in case the reader is in the fast path, pairing
217 	 * with rwbase_read_unlock(), provides ACQUIRE.
218 	 */
219 	if (!atomic_read_acquire(&rwb->readers)) {
220 		atomic_set(&rwb->readers, WRITER_BIAS);
221 		return 1;
222 	}
223 
224 	return 0;
225 }
226 
rwbase_write_lock(struct rwbase_rt * rwb,unsigned int state)227 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
228 				     unsigned int state)
229 {
230 	struct rt_mutex_base *rtm = &rwb->rtmutex;
231 	unsigned long flags;
232 
233 	/* Take the rtmutex as a first step */
234 	if (rwbase_rtmutex_lock_state(rtm, state))
235 		return -EINTR;
236 
237 	/* Force readers into slow path */
238 	atomic_sub(READER_BIAS, &rwb->readers);
239 
240 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
241 	if (__rwbase_write_trylock(rwb))
242 		goto out_unlock;
243 
244 	rwbase_set_and_save_current_state(state);
245 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
246 	for (;;) {
247 		/* Optimized out for rwlocks */
248 		if (rwbase_signal_pending_state(state, current)) {
249 			rwbase_restore_current_state();
250 			__rwbase_write_unlock(rwb, 0, flags);
251 			trace_contention_end(rwb, -EINTR);
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 	trace_contention_end(rwb, 0);
266 
267 out_unlock:
268 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
269 	return 0;
270 }
271 
rwbase_write_trylock(struct rwbase_rt * rwb)272 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
273 {
274 	struct rt_mutex_base *rtm = &rwb->rtmutex;
275 	unsigned long flags;
276 
277 	if (!rwbase_rtmutex_trylock(rtm))
278 		return 0;
279 
280 	atomic_sub(READER_BIAS, &rwb->readers);
281 
282 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
283 	if (__rwbase_write_trylock(rwb)) {
284 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
285 		return 1;
286 	}
287 	__rwbase_write_unlock(rwb, 0, flags);
288 	return 0;
289 }
290