/* * 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> * * This file may be redistributed under the terms of the GNU General Public * License v2. */ #include <linux/list.h>#include <linux/spinlock.h>#include <linux/atomic.h>#include <linux/tty.h>#include <linux/sched.h>  #ifdef CONFIG_DEBUG_LOCK_ALLOC# define __acq(l, s, t, r, c, n, i)		\				lock_acquire(&(l)->dep_map, s, t, r, c, n, i)# define __rel(l, n, i)				\				lock_release(&(l)->dep_map, n, i)# ifdef CONFIG_PROVE_LOCKING#  define lockdep_acquire(l, s, t, i)		__acq(l, s, t, 0, 2, NULL, i)#  define lockdep_acquire_nest(l, s, t, n, i)	__acq(l, s, t, 0, 2, n, i)#  define lockdep_acquire_read(l, s, t, i)	__acq(l, s, t, 1, 2, NULL, i)#  define lockdep_release(l, n, i)		__rel(l, n, i)# else#  define lockdep_acquire(l, s, t, i)		__acq(l, s, t, 0, 1, NULL, i)#  define lockdep_acquire_nest(l, s, t, n, i)	__acq(l, s, t, 0, 1, n, i)#  define lockdep_acquire_read(l, s, t, i)	__acq(l, s, t, 1, 1, NULL, i)#  define lockdep_release(l, n, i)		__rel(l, n, i)# endif#else# define lockdep_acquire(l, s, t, i)		do { } while (0)# define lockdep_acquire_nest(l, s, t, n, i)	do { } while (0)# define lockdep_acquire_read(l, s, t, i)	do { } while (0)# define lockdep_release(l, n, i)		do { } while (0)#endif #ifdef CONFIG_LOCK_STAT# define lock_stat(_lock, stat)		lock_##stat(&(_lock)->dep_map, _RET_IP_)#else# define lock_stat(_lock, stat)		do { } while (0)#endif  #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;}; static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem){	return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);} /* * ldsem_cmpxchg() updates @*old with the last-known sem->count value. * Returns 1 if count was successfully changed; @*old will have @new value. * Returns 0 if count was not changed; @*old will have most recent sem->count */static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem){	long tmp = atomic_long_cmpxchg(&sem->count, *old, new);	if (tmp == *old) {		*old = new;		return 1;	} else {		*old = tmp;		return 0;	}} /* * 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	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 = ldsem_atomic_update(adjust, sem);	do {		if (count > 0)			break;		if (ldsem_cmpxchg(&count, count - adjust, sem))			return;	} while (1); 	list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {		tsk = waiter->task;		smp_mb();		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 = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);	do {		if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)			return 1;		if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))			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;	struct task_struct *tsk = current;	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 are no waiters,	 * and early-out if not */	do {		if (ldsem_cmpxchg(&count, count + adjust, sem))			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 = tsk;	get_task_struct(tsk); 	/* 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_task_state(tsk, TASK_UNINTERRUPTIBLE); 		if (!waiter.task)			break;		if (!timeout)			break;		timeout = schedule_timeout(timeout);	} 	__set_task_state(tsk, 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) {			ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);			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;	struct task_struct *tsk = current;	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 (ldsem_cmpxchg(&count, count + adjust, sem))			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 = tsk; 	set_task_state(tsk, 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_task_state(tsk, TASK_UNINTERRUPTIBLE);		if ((locked = writer_trylock(sem)))			break;	} 	if (!locked)		ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);	list_del(&waiter.list);	raw_spin_unlock_irq(&sem->wait_lock); 	__set_task_state(tsk, TASK_RUNNING); 	/* lock wait may have timed out */	if (!locked)		return NULL;	return sem;}   static inline int __ldsem_down_read_nested(struct ld_semaphore *sem,					   int subclass, long timeout){	long count; 	lockdep_acquire_read(sem, subclass, 0, _RET_IP_); 	count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);	if (count <= 0) {		lock_stat(sem, contended);		if (!down_read_failed(sem, count, timeout)) {			lockdep_release(sem, 1, _RET_IP_);			return 0;		}	}	lock_stat(sem, acquired);	return 1;} static inline int __ldsem_down_write_nested(struct ld_semaphore *sem,					    int subclass, long timeout){	long count; 	lockdep_acquire(sem, subclass, 0, _RET_IP_); 	count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);	if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {		lock_stat(sem, contended);		if (!down_write_failed(sem, count, timeout)) {			lockdep_release(sem, 1, _RET_IP_);			return 0;		}	}	lock_stat(sem, acquired);	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 = sem->count; 	while (count >= 0) {		if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {			lockdep_acquire_read(sem, 0, 1, _RET_IP_);			lock_stat(sem, acquired);			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);} /* * trylock for writing -- returns 1 if successful, 0 if contention */int ldsem_down_write_trylock(struct ld_semaphore *sem){	long count = sem->count; 	while ((count & LDSEM_ACTIVE_MASK) == 0) {		if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {			lockdep_acquire(sem, 0, 1, _RET_IP_);			lock_stat(sem, acquired);			return 1;		}	}	return 0;} /* * release a read lock */void ldsem_up_read(struct ld_semaphore *sem){	long count; 	lockdep_release(sem, 1, _RET_IP_); 	count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);	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; 	lockdep_release(sem, 1, _RET_IP_); 	count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);	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