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 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 } 132 133 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 } 143 144 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 } 167 168 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 } 180 181 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 } 194 195 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 } 203 204 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 } 213 214 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 } 230 231 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 } 279 280 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