// 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;	int ret; 	raw_spin_lock_irq(&rtm->wait_lock);	/*	 * Allow readers, as long as the writer has not completely	 * acquired the semaphore for write.	 */	if (atomic_read(&rwb->readers) != WRITER_BIAS) {		atomic_inc(&rwb->readers);		raw_spin_unlock_irq(&rtm->wait_lock);		return 0;	} 	/*	 * 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.	 */ 	/*	 * For rwlocks this returns 0 unconditionally, so the below	 * !ret conditionals are optimized out.	 */	ret = rwbase_rtmutex_slowlock_locked(rtm, state); 	/*	 * 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);	raw_spin_unlock_irq(&rtm->wait_lock);	if (!ret)		rwbase_rtmutex_unlock(rtm);	return ret;} static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,					    unsigned int state){	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); 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);	if (__rwbase_write_trylock(rwb))		goto out_unlock; 	rwbase_set_and_save_current_state(state);	for (;;) {		/* Optimized out for rwlocks */		if (rwbase_signal_pending_state(state, current)) {			rwbase_restore_current_state();			__rwbase_write_unlock(rwb, 0, flags);			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(); out_unlock:	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);	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;}