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 * Common code shared between RT rw_semaphore and rwlock 45 */ 46 47 static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) 48 { 49 int r; 50 51 /* 52 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is 53 * set. 54 */ 55 for (r = atomic_read(&rwb->readers); r < 0;) { 56 if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1))) 57 return 1; 58 } 59 return 0; 60 } 61 62 static int __sched __rwbase_read_lock(struct rwbase_rt *rwb, 63 unsigned int state) 64 { 65 struct rt_mutex_base *rtm = &rwb->rtmutex; 66 int ret; 67 68 raw_spin_lock_irq(&rtm->wait_lock); 69 /* 70 * Allow readers, as long as the writer has not completely 71 * acquired the semaphore for write. 72 */ 73 if (atomic_read(&rwb->readers) != WRITER_BIAS) { 74 atomic_inc(&rwb->readers); 75 raw_spin_unlock_irq(&rtm->wait_lock); 76 return 0; 77 } 78 79 /* 80 * Call into the slow lock path with the rtmutex->wait_lock 81 * held, so this can't result in the following race: 82 * 83 * Reader1 Reader2 Writer 84 * down_read() 85 * down_write() 86 * rtmutex_lock(m) 87 * wait() 88 * down_read() 89 * unlock(m->wait_lock) 90 * up_read() 91 * wake(Writer) 92 * lock(m->wait_lock) 93 * sem->writelocked=true 94 * unlock(m->wait_lock) 95 * 96 * up_write() 97 * sem->writelocked=false 98 * rtmutex_unlock(m) 99 * down_read() 100 * down_write() 101 * rtmutex_lock(m) 102 * wait() 103 * rtmutex_lock(m) 104 * 105 * That would put Reader1 behind the writer waiting on 106 * Reader2 to call up_read(), which might be unbound. 107 */ 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 return ret; 128 } 129 130 static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb, 131 unsigned int state) 132 { 133 if (rwbase_read_trylock(rwb)) 134 return 0; 135 136 return __rwbase_read_lock(rwb, state); 137 } 138 139 static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb, 140 unsigned int state) 141 { 142 struct rt_mutex_base *rtm = &rwb->rtmutex; 143 struct task_struct *owner; 144 145 raw_spin_lock_irq(&rtm->wait_lock); 146 /* 147 * Wake the writer, i.e. the rtmutex owner. It might release the 148 * rtmutex concurrently in the fast path (due to a signal), but to 149 * clean up rwb->readers it needs to acquire rtm->wait_lock. The 150 * worst case which can happen is a spurious wakeup. 151 */ 152 owner = rt_mutex_owner(rtm); 153 if (owner) 154 wake_up_state(owner, state); 155 156 raw_spin_unlock_irq(&rtm->wait_lock); 157 } 158 159 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, 160 unsigned int state) 161 { 162 /* 163 * rwb->readers can only hit 0 when a writer is waiting for the 164 * active readers to leave the critical section. 165 */ 166 if (unlikely(atomic_dec_and_test(&rwb->readers))) 167 __rwbase_read_unlock(rwb, state); 168 } 169 170 static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, 171 unsigned long flags) 172 { 173 struct rt_mutex_base *rtm = &rwb->rtmutex; 174 175 atomic_add(READER_BIAS - bias, &rwb->readers); 176 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); 177 rwbase_rtmutex_unlock(rtm); 178 } 179 180 static inline void rwbase_write_unlock(struct rwbase_rt *rwb) 181 { 182 struct rt_mutex_base *rtm = &rwb->rtmutex; 183 unsigned long flags; 184 185 raw_spin_lock_irqsave(&rtm->wait_lock, flags); 186 __rwbase_write_unlock(rwb, WRITER_BIAS, flags); 187 } 188 189 static inline void rwbase_write_downgrade(struct rwbase_rt *rwb) 190 { 191 struct rt_mutex_base *rtm = &rwb->rtmutex; 192 unsigned long flags; 193 194 raw_spin_lock_irqsave(&rtm->wait_lock, flags); 195 /* Release it and account current as reader */ 196 __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags); 197 } 198 199 static int __sched rwbase_write_lock(struct rwbase_rt *rwb, 200 unsigned int state) 201 { 202 struct rt_mutex_base *rtm = &rwb->rtmutex; 203 unsigned long flags; 204 205 /* Take the rtmutex as a first step */ 206 if (rwbase_rtmutex_lock_state(rtm, state)) 207 return -EINTR; 208 209 /* Force readers into slow path */ 210 atomic_sub(READER_BIAS, &rwb->readers); 211 212 raw_spin_lock_irqsave(&rtm->wait_lock, flags); 213 /* 214 * set_current_state() for rw_semaphore 215 * current_save_and_set_rtlock_wait_state() for rwlock 216 */ 217 rwbase_set_and_save_current_state(state); 218 219 /* Block until all readers have left the critical section. */ 220 for (; atomic_read(&rwb->readers);) { 221 /* Optimized out for rwlocks */ 222 if (rwbase_signal_pending_state(state, current)) { 223 __set_current_state(TASK_RUNNING); 224 __rwbase_write_unlock(rwb, 0, flags); 225 return -EINTR; 226 } 227 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); 228 229 /* 230 * Schedule and wait for the readers to leave the critical 231 * section. The last reader leaving it wakes the waiter. 232 */ 233 if (atomic_read(&rwb->readers) != 0) 234 rwbase_schedule(); 235 set_current_state(state); 236 raw_spin_lock_irqsave(&rtm->wait_lock, flags); 237 } 238 239 atomic_set(&rwb->readers, WRITER_BIAS); 240 rwbase_restore_current_state(); 241 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); 242 return 0; 243 } 244 245 static inline int rwbase_write_trylock(struct rwbase_rt *rwb) 246 { 247 struct rt_mutex_base *rtm = &rwb->rtmutex; 248 unsigned long flags; 249 250 if (!rwbase_rtmutex_trylock(rtm)) 251 return 0; 252 253 atomic_sub(READER_BIAS, &rwb->readers); 254 255 raw_spin_lock_irqsave(&rtm->wait_lock, flags); 256 if (!atomic_read(&rwb->readers)) { 257 atomic_set(&rwb->readers, WRITER_BIAS); 258 raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); 259 return 1; 260 } 261 __rwbase_write_unlock(rwb, 0, flags); 262 return 0; 263 } 264