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