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 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 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 } 139 140 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 } 148 149 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 } 172 173 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 } 185 186 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 } 199 200 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 } 208 209 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 } 218 219 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 } 235 236 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 } 280 281 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