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 /* 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 } 130 131 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 } 139 140 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 } 163 164 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 } 176 177 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 } 190 191 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 } 199 200 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 } 209 210 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 } 226 227 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 } 271 272 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