// SPDX-License-Identifier: GPL-2.0/* * Ldisc rw semaphore * * The ldisc semaphore is semantically a rw_semaphore but which enforces * an alternate policy, namely: *   1) Supports lock wait timeouts *   2) Write waiter has priority *   3) Downgrading is not supported * * Implementation notes: *   1) Upper half of semaphore count is a wait count (differs from rwsem *	in that rwsem normalizes the upper half to the wait bias) *   2) Lacks overflow checking * * The generic counting was copied and modified from include/asm-generic/rwsem.h * by Paul Mackerras <paulus@samba.org>. * * The scheduling policy was copied and modified from lib/rwsem.c * Written by David Howells (dhowells@redhat.com). * * This implementation incorporates the write lock stealing work of * Michel Lespinasse <walken@google.com>. * * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com> */ #include <linux/list.h>#include <linux/spinlock.h>#include <linux/atomic.h>#include <linux/tty.h>#include <linux/sched.h>#include <linux/sched/debug.h>#include <linux/sched/task.h>  #if BITS_PER_LONG == 64# define LDSEM_ACTIVE_MASK	0xffffffffL#else# define LDSEM_ACTIVE_MASK	0x0000ffffL#endif #define LDSEM_UNLOCKED		0L#define LDSEM_ACTIVE_BIAS	1L#define LDSEM_WAIT_BIAS		(-LDSEM_ACTIVE_MASK-1)#define LDSEM_READ_BIAS		LDSEM_ACTIVE_BIAS#define LDSEM_WRITE_BIAS	(LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) struct ldsem_waiter {	struct list_head list;	struct task_struct *task;}; /* * Initialize an ldsem: */void __init_ldsem(struct ld_semaphore *sem, const char *name,		  struct lock_class_key *key){#ifdef CONFIG_DEBUG_LOCK_ALLOC	/*	 * Make sure we are not reinitializing a held semaphore:	 */	debug_check_no_locks_freed((void *)sem, sizeof(*sem));	lockdep_init_map(&sem->dep_map, name, key, 0);#endif	atomic_long_set(&sem->count, LDSEM_UNLOCKED);	sem->wait_readers = 0;	raw_spin_lock_init(&sem->wait_lock);	INIT_LIST_HEAD(&sem->read_wait);	INIT_LIST_HEAD(&sem->write_wait);} static void __ldsem_wake_readers(struct ld_semaphore *sem){	struct ldsem_waiter *waiter, *next;	struct task_struct *tsk;	long adjust, count; 	/*	 * Try to grant read locks to all readers on the read wait list.	 * Note the 'active part' of the count is incremented by	 * the number of readers before waking any processes up.	 */	adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);	count = atomic_long_add_return(adjust, &sem->count);	do {		if (count > 0)			break;		if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust))			return;	} while (1); 	list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {		tsk = waiter->task;		smp_store_release(&waiter->task, NULL);		wake_up_process(tsk);		put_task_struct(tsk);	}	INIT_LIST_HEAD(&sem->read_wait);	sem->wait_readers = 0;} static inline int writer_trylock(struct ld_semaphore *sem){	/*	 * Only wake this writer if the active part of the count can be	 * transitioned from 0 -> 1	 */	long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count);	do {		if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)			return 1;		if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS))			return 0;	} while (1);} static void __ldsem_wake_writer(struct ld_semaphore *sem){	struct ldsem_waiter *waiter; 	waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);	wake_up_process(waiter->task);} /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) * - the spinlock must be held by the caller * - woken process blocks are discarded from the list after having task zeroed */static void __ldsem_wake(struct ld_semaphore *sem){	if (!list_empty(&sem->write_wait))		__ldsem_wake_writer(sem);	else if (!list_empty(&sem->read_wait))		__ldsem_wake_readers(sem);} static void ldsem_wake(struct ld_semaphore *sem){	unsigned long flags; 	raw_spin_lock_irqsave(&sem->wait_lock, flags);	__ldsem_wake(sem);	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);} /* * wait for the read lock to be granted */static struct ld_semaphore __sched *down_read_failed(struct ld_semaphore *sem, long count, long timeout){	struct ldsem_waiter waiter;	long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; 	/* set up my own style of waitqueue */	raw_spin_lock_irq(&sem->wait_lock); 	/*	 * Try to reverse the lock attempt but if the count has changed	 * so that reversing fails, check if there are no waiters,	 * and early-out if not	 */	do {		if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) {			count += adjust;			break;		}		if (count > 0) {			raw_spin_unlock_irq(&sem->wait_lock);			return sem;		}	} while (1); 	list_add_tail(&waiter.list, &sem->read_wait);	sem->wait_readers++; 	waiter.task = current;	get_task_struct(current); 	/* if there are no active locks, wake the new lock owner(s) */	if ((count & LDSEM_ACTIVE_MASK) == 0)		__ldsem_wake(sem); 	raw_spin_unlock_irq(&sem->wait_lock); 	/* wait to be given the lock */	for (;;) {		set_current_state(TASK_UNINTERRUPTIBLE); 		if (!smp_load_acquire(&waiter.task))			break;		if (!timeout)			break;		timeout = schedule_timeout(timeout);	} 	__set_current_state(TASK_RUNNING); 	if (!timeout) {		/*		 * Lock timed out but check if this task was just		 * granted lock ownership - if so, pretend there		 * was no timeout; otherwise, cleanup lock wait.		 */		raw_spin_lock_irq(&sem->wait_lock);		if (waiter.task) {			atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);			sem->wait_readers--;			list_del(&waiter.list);			raw_spin_unlock_irq(&sem->wait_lock);			put_task_struct(waiter.task);			return NULL;		}		raw_spin_unlock_irq(&sem->wait_lock);	} 	return sem;} /* * wait for the write lock to be granted */static struct ld_semaphore __sched *down_write_failed(struct ld_semaphore *sem, long count, long timeout){	struct ldsem_waiter waiter;	long adjust = -LDSEM_ACTIVE_BIAS;	int locked = 0; 	/* set up my own style of waitqueue */	raw_spin_lock_irq(&sem->wait_lock); 	/*	 * Try to reverse the lock attempt but if the count has changed	 * so that reversing fails, check if the lock is now owned,	 * and early-out if so.	 */	do {		if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust))			break;		if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {			raw_spin_unlock_irq(&sem->wait_lock);			return sem;		}	} while (1); 	list_add_tail(&waiter.list, &sem->write_wait); 	waiter.task = current; 	set_current_state(TASK_UNINTERRUPTIBLE);	for (;;) {		if (!timeout)			break;		raw_spin_unlock_irq(&sem->wait_lock);		timeout = schedule_timeout(timeout);		raw_spin_lock_irq(&sem->wait_lock);		set_current_state(TASK_UNINTERRUPTIBLE);		locked = writer_trylock(sem);		if (locked)			break;	} 	if (!locked)		atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);	list_del(&waiter.list); 	/*	 * In case of timeout, wake up every reader who gave the right of way	 * to writer. Prevent separation readers into two groups:	 * one that helds semaphore and another that sleeps.	 * (in case of no contention with a writer)	 */	if (!locked && list_empty(&sem->write_wait))		__ldsem_wake_readers(sem); 	raw_spin_unlock_irq(&sem->wait_lock); 	__set_current_state(TASK_RUNNING); 	/* lock wait may have timed out */	if (!locked)		return NULL;	return sem;}   static int __ldsem_down_read_nested(struct ld_semaphore *sem,					   int subclass, long timeout){	long count; 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_); 	count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count);	if (count <= 0) {		lock_contended(&sem->dep_map, _RET_IP_);		if (!down_read_failed(sem, count, timeout)) {			rwsem_release(&sem->dep_map, _RET_IP_);			return 0;		}	}	lock_acquired(&sem->dep_map, _RET_IP_);	return 1;} static int __ldsem_down_write_nested(struct ld_semaphore *sem,					    int subclass, long timeout){	long count; 	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_); 	count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count);	if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {		lock_contended(&sem->dep_map, _RET_IP_);		if (!down_write_failed(sem, count, timeout)) {			rwsem_release(&sem->dep_map, _RET_IP_);			return 0;		}	}	lock_acquired(&sem->dep_map, _RET_IP_);	return 1;}  /* * lock for reading -- returns 1 if successful, 0 if timed out */int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout){	might_sleep();	return __ldsem_down_read_nested(sem, 0, timeout);} /* * trylock for reading -- returns 1 if successful, 0 if contention */int ldsem_down_read_trylock(struct ld_semaphore *sem){	long count = atomic_long_read(&sem->count); 	while (count >= 0) {		if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) {			rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);			lock_acquired(&sem->dep_map, _RET_IP_);			return 1;		}	}	return 0;} /* * lock for writing -- returns 1 if successful, 0 if timed out */int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout){	might_sleep();	return __ldsem_down_write_nested(sem, 0, timeout);} /* * release a read lock */void ldsem_up_read(struct ld_semaphore *sem){	long count; 	rwsem_release(&sem->dep_map, _RET_IP_); 	count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);	if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)		ldsem_wake(sem);} /* * release a write lock */void ldsem_up_write(struct ld_semaphore *sem){	long count; 	rwsem_release(&sem->dep_map, _RET_IP_); 	count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);	if (count < 0)		ldsem_wake(sem);}  #ifdef CONFIG_DEBUG_LOCK_ALLOC int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout){	might_sleep();	return __ldsem_down_read_nested(sem, subclass, timeout);} int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,			    long timeout){	might_sleep();	return __ldsem_down_write_nested(sem, subclass, timeout);} #endif