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 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 } 138 139 static __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 } 149 150 static 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 } 173 174 static __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 } 186 187 static 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 } 200 201 static 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, WRITER_BIAS, flags); 208 } 209 210 static 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, WRITER_BIAS - 1, flags); 218 } 219 220 static 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 } 236 237 static 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); 279 280 out_unlock: 281 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); 282 rwbase_post_schedule(); 283 return 0; 284 } 285 286 static 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, 0, flags); 302 return 0; 303 } 304