locking/rt: Add base code for RT rw_semaphore and rwlock

// SPDX-License-Identifier: GPL-2.0-only

#ifndef _LINUX_RWBASE_RT_H

#define _LINUX_RWBASE_RT_H

#include <linux/rtmutex.h>

#include <linux/atomic.h>

#define READER_BIAS		(1U << 31)

#define WRITER_BIAS		(1U << 30)

struct rwbase_rt {

	atomic_t		readers;

	struct rt_mutex_base	rtmutex;

};

#define __RWBASE_INITIALIZER(name)				\

{								\

	.readers = ATOMIC_INIT(READER_BIAS),			\

	.rtmutex = __RT_MUTEX_BASE_INITIALIZER(name.rtmutex),	\

}

#define init_rwbase_rt(rwbase)					\

	do {							\

		rt_mutex_base_init(&(rwbase)->rtmutex);		\

		atomic_set(&(rwbase)->readers, READER_BIAS);	\

	} while (0)

static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb)

{

	return atomic_read(&rwb->readers) != READER_BIAS;

}

static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb)

{

	return atomic_read(&rwb->readers) > 0;

}

#endif /* _LINUX_RWBASE_RT_H */

// 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.

 *

 * 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(&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;

	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)

		wake_up_state(owner, state);

	raw_spin_unlock_irq(&rtm->wait_lock);

}

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.

	 */

	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;

	atomic_add(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 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);

	/*

	 * set_current_state() for rw_semaphore

	 * current_save_and_set_rtlock_wait_state() for rwlock

	 */

	rwbase_set_and_save_current_state(state);

	/* Block until all readers have left the critical section. */

	for (; atomic_read(&rwb->readers);) {

		/* Optimized out for rwlocks */

		if (rwbase_signal_pending_state(state, current)) {

			__set_current_state(TASK_RUNNING);

			__rwbase_write_unlock(rwb, 0, flags);

			return -EINTR;

		}

		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);

		/*

		 * Schedule and wait for the readers to leave the critical

		 * section. The last reader leaving it wakes the waiter.

		 */

		if (atomic_read(&rwb->readers) != 0)

			rwbase_schedule();

		set_current_state(state);

		raw_spin_lock_irqsave(&rtm->wait_lock, flags);

	}

	atomic_set(&rwb->readers, WRITER_BIAS);

	rwbase_restore_current_state();

	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 (!atomic_read(&rwb->readers)) {

		atomic_set(&rwb->readers, WRITER_BIAS);

		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);

		return 1;

	}

	__rwbase_write_unlock(rwb, 0, flags);

	return 0;

}