xref: /linux/kernel/locking/rwbase_rt.c (revision 0ea5c948cb64bab5bc7a5516774eb8536f05aa0d)
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 	rwbase_pre_schedule();
75 	raw_spin_lock_irq(&rtm->wait_lock);
76 
77 	/*
78 	 * Call into the slow lock path with the rtmutex->wait_lock
79 	 * held, so this can't result in the following race:
80 	 *
81 	 * Reader1		Reader2		Writer
82 	 *			down_read()
83 	 *					down_write()
84 	 *					rtmutex_lock(m)
85 	 *					wait()
86 	 * down_read()
87 	 * unlock(m->wait_lock)
88 	 *			up_read()
89 	 *			wake(Writer)
90 	 *					lock(m->wait_lock)
91 	 *					sem->writelocked=true
92 	 *					unlock(m->wait_lock)
93 	 *
94 	 *					up_write()
95 	 *					sem->writelocked=false
96 	 *					rtmutex_unlock(m)
97 	 *			down_read()
98 	 *					down_write()
99 	 *					rtmutex_lock(m)
100 	 *					wait()
101 	 * rtmutex_lock(m)
102 	 *
103 	 * That would put Reader1 behind the writer waiting on
104 	 * Reader2 to call up_read(), which might be unbound.
105 	 */
106 
107 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
108 
109 	/*
110 	 * For rwlocks this returns 0 unconditionally, so the below
111 	 * !ret conditionals are optimized out.
112 	 */
113 	ret = rwbase_rtmutex_slowlock_locked(rtm, state);
114 
115 	/*
116 	 * On success the rtmutex is held, so there can't be a writer
117 	 * active. Increment the reader count and immediately drop the
118 	 * rtmutex again.
119 	 *
120 	 * rtmutex->wait_lock has to be unlocked in any case of course.
121 	 */
122 	if (!ret)
123 		atomic_inc(&rwb->readers);
124 	raw_spin_unlock_irq(&rtm->wait_lock);
125 	if (!ret)
126 		rwbase_rtmutex_unlock(rtm);
127 
128 	trace_contention_end(rwb, ret);
129 	rwbase_post_schedule();
130 	return ret;
131 }
132 
rwbase_read_lock(struct rwbase_rt * rwb,unsigned int state)133 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
134 					    unsigned int state)
135 {
136 	lockdep_assert(!current->pi_blocked_on);
137 
138 	if (rwbase_read_trylock(rwb))
139 		return 0;
140 
141 	return __rwbase_read_lock(rwb, state);
142 }
143 
__rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)144 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
145 					 unsigned int state)
146 {
147 	struct rt_mutex_base *rtm = &rwb->rtmutex;
148 	struct task_struct *owner;
149 	DEFINE_RT_WAKE_Q(wqh);
150 
151 	raw_spin_lock_irq(&rtm->wait_lock);
152 	/*
153 	 * Wake the writer, i.e. the rtmutex owner. It might release the
154 	 * rtmutex concurrently in the fast path (due to a signal), but to
155 	 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
156 	 * worst case which can happen is a spurious wakeup.
157 	 */
158 	owner = rt_mutex_owner(rtm);
159 	if (owner)
160 		rt_mutex_wake_q_add_task(&wqh, owner, state);
161 
162 	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
163 	preempt_disable();
164 	raw_spin_unlock_irq(&rtm->wait_lock);
165 	rt_mutex_wake_up_q(&wqh);
166 }
167 
rwbase_read_unlock(struct rwbase_rt * rwb,unsigned int state)168 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
169 					       unsigned int state)
170 {
171 	/*
172 	 * rwb->readers can only hit 0 when a writer is waiting for the
173 	 * active readers to leave the critical section.
174 	 *
175 	 * dec_and_test() is fully ordered, provides RELEASE.
176 	 */
177 	if (unlikely(atomic_dec_and_test(&rwb->readers)))
178 		__rwbase_read_unlock(rwb, state);
179 }
180 
__rwbase_write_unlock(struct rwbase_rt * rwb,int bias,unsigned long flags)181 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
182 					 unsigned long flags)
183 {
184 	struct rt_mutex_base *rtm = &rwb->rtmutex;
185 
186 	/*
187 	 * _release() is needed in case that reader is in fast path, pairing
188 	 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
189 	 */
190 	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
191 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
192 	rwbase_rtmutex_unlock(rtm);
193 }
194 
rwbase_write_unlock(struct rwbase_rt * rwb)195 static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
196 {
197 	struct rt_mutex_base *rtm = &rwb->rtmutex;
198 	unsigned long flags;
199 
200 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
201 	__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
202 }
203 
rwbase_write_downgrade(struct rwbase_rt * rwb)204 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
205 {
206 	struct rt_mutex_base *rtm = &rwb->rtmutex;
207 	unsigned long flags;
208 
209 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
210 	/* Release it and account current as reader */
211 	__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
212 }
213 
__rwbase_write_trylock(struct rwbase_rt * rwb)214 static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
215 {
216 	/* Can do without CAS because we're serialized by wait_lock. */
217 	lockdep_assert_held(&rwb->rtmutex.wait_lock);
218 
219 	/*
220 	 * _acquire is needed in case the reader is in the fast path, pairing
221 	 * with rwbase_read_unlock(), provides ACQUIRE.
222 	 */
223 	if (!atomic_read_acquire(&rwb->readers)) {
224 		atomic_set(&rwb->readers, WRITER_BIAS);
225 		return 1;
226 	}
227 
228 	return 0;
229 }
230 
rwbase_write_lock(struct rwbase_rt * rwb,unsigned int state)231 static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
232 				     unsigned int state)
233 {
234 	struct rt_mutex_base *rtm = &rwb->rtmutex;
235 	unsigned long flags;
236 
237 	/* Take the rtmutex as a first step */
238 	if (rwbase_rtmutex_lock_state(rtm, state))
239 		return -EINTR;
240 
241 	/* Force readers into slow path */
242 	atomic_sub(READER_BIAS, &rwb->readers);
243 
244 	rwbase_pre_schedule();
245 
246 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
247 	if (__rwbase_write_trylock(rwb))
248 		goto out_unlock;
249 
250 	rwbase_set_and_save_current_state(state);
251 	trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
252 	for (;;) {
253 		/* Optimized out for rwlocks */
254 		if (rwbase_signal_pending_state(state, current)) {
255 			rwbase_restore_current_state();
256 			__rwbase_write_unlock(rwb, 0, flags);
257 			rwbase_post_schedule();
258 			trace_contention_end(rwb, -EINTR);
259 			return -EINTR;
260 		}
261 
262 		if (__rwbase_write_trylock(rwb))
263 			break;
264 
265 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
266 		rwbase_schedule();
267 		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
268 
269 		set_current_state(state);
270 	}
271 	rwbase_restore_current_state();
272 	trace_contention_end(rwb, 0);
273 
274 out_unlock:
275 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
276 	rwbase_post_schedule();
277 	return 0;
278 }
279 
rwbase_write_trylock(struct rwbase_rt * rwb)280 static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
281 {
282 	struct rt_mutex_base *rtm = &rwb->rtmutex;
283 	unsigned long flags;
284 
285 	if (!rwbase_rtmutex_trylock(rtm))
286 		return 0;
287 
288 	atomic_sub(READER_BIAS, &rwb->readers);
289 
290 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
291 	if (__rwbase_write_trylock(rwb)) {
292 		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
293 		return 1;
294 	}
295 	__rwbase_write_unlock(rwb, 0, flags);
296 	return 0;
297 }
298