path: root/lib/librthread/rthread_file.c

/*	$OpenBSD: rthread_file.c,v 1.4 2012/01/17 02:34:18 guenther Exp $	*/
/*
 * Copyright (c) 1995 John Birrell <jb@cimlogic.com.au>.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by John Birrell.
 * 4. Neither the name of the author nor the names of any co-contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY JOHN BIRRELL AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD: uthread_file.c,v 1.9 1999/08/28 00:03:32 peter Exp $
 *
 * POSIX stdio FILE locking functions. These assume that the locking
 * is only required at FILE structure level, not at file descriptor
 * level too.
 *
 */
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
#include <pthread.h>
#include "rthread.h"

/*
 * The FILE lock structure. The FILE *fp is locked if the owner is
 * not NULL. If not locked, the file lock structure can be
 * reassigned to a different file by setting fp.
 */
struct	file_lock {
	LIST_ENTRY(file_lock)	entry;	/* Entry if file list.       */
	FILE			*fp;	/* The target file.          */
	struct pthread_queue	lockers;
	pthread_t		owner;
	int			count;
};

/*
 * The number of file lock lists into which the file pointer is
 * hashed. Ideally, the FILE structure size would have been increased,
 * but this causes incompatibility, so separate data structures are
 * required.
 */
#define NUM_HEADS	128

/*
 * This macro casts a file pointer to a long integer and right
 * shifts this by the number of bytes in a pointer. The shifted
 * value is then remaindered using the maximum number of hash
 * entries to produce and index into the array of static lock
 * structures. If there is a collision, a linear search of the
 * dynamic list of locks linked to each static lock is perfomed.
 */
#define file_idx(_p)	((int)((((uintptr_t) _p) >> sizeof(void *)) % NUM_HEADS))

/*
 * Global array of file locks. The first lock for each hash bucket is
 * allocated statically in the hope that there won't be too many
 * collisions that require a malloc and an element added to the list.
 */
static struct static_file_lock {
	LIST_HEAD(file_list_head, file_lock) head;
	struct	file_lock	fl;
} flh[NUM_HEADS];

/* Lock for accesses to the hash table: */
static	_spinlock_lock_t	hash_lock	= _SPINLOCK_UNLOCKED;

/*
 * Find a lock structure for a FILE, return NULL if the file is
 * not locked:
 */
static
struct file_lock *
find_lock(int idx, FILE *fp)
{
	struct file_lock *p;

	/* Check if the file is locked using the static structure: */
	if (flh[idx].fl.fp == fp && flh[idx].fl.owner != NULL)
		/* Return a pointer to the static lock: */
		p = &flh[idx].fl;
	else {
		/* Point to the first dynamic lock: */
		p = LIST_FIRST(&flh[idx].head);

		/*
		 * Loop through the dynamic locks looking for the
		 * target file:
		 */
		while (p != NULL && (p->fp != fp || p->owner == NULL))
			/* Not this file, try the next: */
			p = LIST_NEXT(p, entry);
	}
	return(p);
}

/*
 * Lock a file, assuming that there is no lock structure currently
 * assigned to it.
 */
static
struct file_lock *
do_lock(int idx, FILE *fp)
{
	struct file_lock *p;

	/* Check if the static structure is not being used: */
	if (flh[idx].fl.owner == NULL) {
		/* Return a pointer to the static lock: */
		p = &flh[idx].fl;
	}
	else {
		/* Point to the first dynamic lock: */
		p = LIST_FIRST(&flh[idx].head);

		/*
		 * Loop through the dynamic locks looking for a
		 * lock structure that is not being used:
		 */
		while (p != NULL && p->owner != NULL)
			/* This one is used, try the next: */
			p = LIST_NEXT(p, entry);
	}

	/*
	 * If an existing lock structure has not been found,
	 * allocate memory for a new one:
	 */
	if (p == NULL && (p = (struct file_lock *)
	    malloc(sizeof(struct file_lock))) != NULL) {
		/* Add the new element to the list: */
		LIST_INSERT_HEAD(&flh[idx].head, p, entry);
	}

	/* Check if there is a lock structure to acquire: */
	if (p != NULL) {
		/* Acquire the lock for the running thread: */
		p->fp		= fp;
		p->owner	= pthread_self();
		p->count	= 1;
		TAILQ_INIT(&p->lockers);
	}
	return(p);
}

void
(flockfile)(FILE * fp)
{
	int	idx = file_idx(fp);
	struct	file_lock	*p;
	pthread_t	self = pthread_self();

	/* Lock the hash table: */
	_spinlock(&hash_lock);

	/* Get a pointer to any existing lock for the file: */
	if ((p = find_lock(idx, fp)) == NULL) {
		/*
		 * The file is not locked, so this thread can
		 * grab the lock:
		 */
		p = do_lock(idx, fp);

	/*
	 * The file is already locked, so check if the
	 * running thread is the owner:
	 */
	} else if (p->owner == self) {
		/*
		 * The running thread is already the
		 * owner, so increment the count of
		 * the number of times it has locked
		 * the file:
		 */
		p->count++;
	} else {
		/*
		 * The file is locked for another thread.
		 * Append this thread to the queue of
		 * threads waiting on the lock.
		 */
		TAILQ_INSERT_TAIL(&p->lockers,self,waiting);
		while (p->owner != self) {
			__thrsleep(self, 0, NULL, &hash_lock, NULL);
			_spinlock(&hash_lock);
		}
	}

	/* Unlock the hash table: */
	_spinunlock(&hash_lock);
}

int
(ftrylockfile)(FILE * fp)
{
	int	ret = -1;
	int	idx = file_idx(fp);
	struct	file_lock	*p;

	/* Lock the hash table: */
	_spinlock(&hash_lock);

	/* Get a pointer to any existing lock for the file: */
	if ((p = find_lock(idx, fp)) == NULL) {
		/*
		 * The file is not locked, so this thread can
		 * grab the lock:
		 */
		p = do_lock(idx, fp);

	/*
	 * The file is already locked, so check if the
	 * running thread is the owner:
	 */
	} else if (p->owner == pthread_self()) {
		/*
		 * The running thread is already the
		 * owner, so increment the count of
		 * the number of times it has locked
		 * the file:
		 */
		p->count++;
	} else {
		/*
		 * The file is locked for another thread,
		 * so this try fails.
		 */
		p = NULL;
	}

	/* Unlock the hash table: */
	_spinunlock(&hash_lock);

	/* Check if the lock was obtained: */
	if (p != NULL)
		/* Return success: */
		ret = 0;

	return (ret);
}

void 
(funlockfile)(FILE * fp)
{
	int	idx = file_idx(fp);
	struct	file_lock	*p;

	/* Lock the hash table: */
	_spinlock(&hash_lock);

	/*
	 * Get a pointer to the lock for the file and check that
	 * the running thread is the one with the lock:
	 */
	if ((p = find_lock(idx, fp)) != NULL && p->owner == pthread_self()) {
		/*
		 * Check if this thread has locked the FILE
		 * more than once:
		 */
		if (--p->count == 0) {
			/* Get the new owner of the lock: */
			if ((p->owner = TAILQ_FIRST(&p->lockers)) != NULL) {
				/* Pop the thread off the queue: */
				TAILQ_REMOVE(&p->lockers,p->owner,waiting);

				/*
				 * This is the first lock for the new
				 * owner:
				 */
				p->count = 1;

				__thrwakeup(p->owner, 1);
			}
		}
	}

	/* Unlock the hash table: */
	_spinunlock(&hash_lock);
}