// SPDX-License-Identifier: GPL-2.0-only /* * RT-specific reader/writer semaphores and reader/writer locks * * down_write/write_lock() * 1) Lock rtmutex * 2) Remove the reader BIAS to force readers into the slow path * 3) Wait until all readers have left the critical section * 4) Mark it write locked * * up_write/write_unlock() * 1) Remove the write locked marker * 2) Set the reader BIAS, so readers can use the fast path again * 3) Unlock rtmutex, to release blocked readers * * down_read/read_lock() * 1) Try fast path acquisition (reader BIAS is set) * 2) Take tmutex::wait_lock, which protects the writelocked flag * 3) If !writelocked, acquire it for read * 4) If writelocked, block on tmutex * 5) unlock rtmutex, goto 1) * * up_read/read_unlock() * 1) Try fast path release (reader count != 1) * 2) Wake the writer waiting in down_write()/write_lock() #3 * * down_read/read_lock()#3 has the consequence, that rw semaphores and rw * locks on RT are not writer fair, but writers, which should be avoided in * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL * inheritance mechanism. * * It's possible to make the rw primitives writer fair by keeping a list of * active readers. A blocked writer would force all newly incoming readers * to block on the rtmutex, but the rtmutex would have to be proxy locked * for one reader after the other. We can't use multi-reader inheritance * because there is no way to support that with SCHED_DEADLINE. * Implementing the one by one reader boosting/handover mechanism is a * major surgery for a very dubious value. * * The risk of writer starvation is there, but the pathological use cases * which trigger it are not necessarily the typical RT workloads. * * Fast-path orderings: * The lock/unlock of readers can run in fast paths: lock and unlock are only * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE * semantics of rwbase_rt. Atomic ops should thus provide _acquire() * and _release() (or stronger). * * Common code shared between RT rw_semaphore and rwlock */ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) { int r; /* * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is * set. */ for (r = atomic_read(&rwb->readers); r < 0;) { if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1))) return 1; } return 0; } static int __sched __rwbase_read_lock(struct rwbase_rt *rwb, unsigned int state) { struct rt_mutex_base *rtm = &rwb->rtmutex; DEFINE_WAKE_Q(wake_q); int ret; rwbase_pre_schedule(); raw_spin_lock_irq(&rtm->wait_lock); /* * Call into the slow lock path with the rtmutex->wait_lock * held, so this can't result in the following race: * * Reader1 Reader2 Writer * down_read() * down_write() * rtmutex_lock(m) * wait() * down_read() * unlock(m->wait_lock) * up_read() * wake(Writer) * lock(m->wait_lock) * sem->writelocked=true * unlock(m->wait_lock) * * up_write() * sem->writelocked=false * rtmutex_unlock(m) * down_read() * down_write() * rtmutex_lock(m) * wait() * rtmutex_lock(m) * * That would put Reader1 behind the writer waiting on * Reader2 to call up_read(), which might be unbound. */ trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ); /* * For rwlocks this returns 0 unconditionally, so the below * !ret conditionals are optimized out. */ ret = rwbase_rtmutex_slowlock_locked(rtm, state, &wake_q); /* * On success the rtmutex is held, so there can't be a writer * active. Increment the reader count and immediately drop the * rtmutex again. * * rtmutex->wait_lock has to be unlocked in any case of course. */ if (!ret) atomic_inc(&rwb->readers); preempt_disable(); raw_spin_unlock_irq(&rtm->wait_lock); wake_up_q(&wake_q); preempt_enable(); if (!ret) rwbase_rtmutex_unlock(rtm); trace_contention_end(rwb, ret); rwbase_post_schedule(); return ret; } static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb, unsigned int state) { lockdep_assert(!current->pi_blocked_on); if (rwbase_read_trylock(rwb)) return 0; return __rwbase_read_lock(rwb, state); } static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb, unsigned int state) { struct rt_mutex_base *rtm = &rwb->rtmutex; struct task_struct *owner; DEFINE_RT_WAKE_Q(wqh); raw_spin_lock_irq(&rtm->wait_lock); /* * Wake the writer, i.e. the rtmutex owner. It might release the * rtmutex concurrently in the fast path (due to a signal), but to * clean up rwb->readers it needs to acquire rtm->wait_lock. The * worst case which can happen is a spurious wakeup. */ owner = rt_mutex_owner(rtm); if (owner) rt_mutex_wake_q_add_task(&wqh, owner, state); /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */ preempt_disable(); raw_spin_unlock_irq(&rtm->wait_lock); rt_mutex_wake_up_q(&wqh); } static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb, unsigned int state) { /* * rwb->readers can only hit 0 when a writer is waiting for the * active readers to leave the critical section. * * dec_and_test() is fully ordered, provides RELEASE. */ if (unlikely(atomic_dec_and_test(&rwb->readers))) __rwbase_read_unlock(rwb, state); } static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, unsigned long flags) { struct rt_mutex_base *rtm = &rwb->rtmutex; /* * _release() is needed in case that reader is in fast path, pairing * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock(). */ (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers); raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); rwbase_rtmutex_unlock(rtm); } static inline void rwbase_write_unlock(struct rwbase_rt *rwb) { struct rt_mutex_base *rtm = &rwb->rtmutex; unsigned long flags; raw_spin_lock_irqsave(&rtm->wait_lock, flags); __rwbase_write_unlock(rwb, WRITER_BIAS, flags); } static inline void rwbase_write_downgrade(struct rwbase_rt *rwb) { struct rt_mutex_base *rtm = &rwb->rtmutex; unsigned long flags; raw_spin_lock_irqsave(&rtm->wait_lock, flags); /* Release it and account current as reader */ __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags); } static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb) { /* Can do without CAS because we're serialized by wait_lock. */ lockdep_assert_held(&rwb->rtmutex.wait_lock); /* * _acquire is needed in case the reader is in the fast path, pairing * with rwbase_read_unlock(), provides ACQUIRE. */ if (!atomic_read_acquire(&rwb->readers)) { atomic_set(&rwb->readers, WRITER_BIAS); return 1; } return 0; } static int __sched rwbase_write_lock(struct rwbase_rt *rwb, unsigned int state) { struct rt_mutex_base *rtm = &rwb->rtmutex; unsigned long flags; /* Take the rtmutex as a first step */ if (rwbase_rtmutex_lock_state(rtm, state)) return -EINTR; /* Force readers into slow path */ atomic_sub(READER_BIAS, &rwb->readers); rwbase_pre_schedule(); raw_spin_lock_irqsave(&rtm->wait_lock, flags); if (__rwbase_write_trylock(rwb)) goto out_unlock; rwbase_set_and_save_current_state(state); trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE); for (;;) { /* Optimized out for rwlocks */ if (rwbase_signal_pending_state(state, current)) { rwbase_restore_current_state(); __rwbase_write_unlock(rwb, 0, flags); rwbase_post_schedule(); trace_contention_end(rwb, -EINTR); return -EINTR; } if (__rwbase_write_trylock(rwb)) break; raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); rwbase_schedule(); raw_spin_lock_irqsave(&rtm->wait_lock, flags); set_current_state(state); } rwbase_restore_current_state(); trace_contention_end(rwb, 0); out_unlock: raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); rwbase_post_schedule(); return 0; } static inline int rwbase_write_trylock(struct rwbase_rt *rwb) { struct rt_mutex_base *rtm = &rwb->rtmutex; unsigned long flags; if (!rwbase_rtmutex_trylock(rtm)) return 0; atomic_sub(READER_BIAS, &rwb->readers); raw_spin_lock_irqsave(&rtm->wait_lock, flags); if (__rwbase_write_trylock(rwb)) { raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); return 1; } __rwbase_write_unlock(rwb, 0, flags); return 0; }
