Error using the buffer cache

mcatos
2007-05-03T13:32:14Z

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I've written a module that acts as a cache for fixed size objects but I'm
getting a soft lockup using the buffer cache as backing storage.

I've attached the code that reproduces the error. You need to supply the module
with a valid block device, i.e. insmod disk_cache.ko devname="/dev/hda2".

Thanx.

P.S: It's the first time I'm writing here, please be gentle...

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/*
* An object oriented disk cache.
*/

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/types.h>
#include <linux/vmalloc.h>
#include <linux/genhd.h>
#include <linux/blkdev.h>
#include <linux/hdreg.h>
#include <linux/workqueue.h>
#include <linux/bio.h>
#include <linux/buffer_head.h>
//#include "assert.h"
//#include "debug.h"
#define assert(s) if(!(s)) { printk(KERN_EMERG "assertion failed: %d: ", __LINE__); panic(#s);}

#define KERNEL_SECTOR_SIZE 512

MODULE_LICENSE("Dual BSD/GPL");

struct disk_cache {
int object_size; /* object size */
sector_t cache_size; /* number of objects to be held in cache */
sector_t disk_size; /* maximum number of objects */
int objects_per_sector; /* number of objects per sector */
sector_t start; /* start sector for data storage */
};

static int major_num = 0;

struct block_device *bdev = NULL;

sector_t start = 0;

static char *devname = NULL;
module_param(devname, charp, 0);

/*
* Create and initialize a disk cache.
* @disk_cache - an allocated disk_cache structure
* @object_size - the object size to use (in bytes)
* @cache_size - total memory size for cache (in objects)
* @disk_size - total disk size for objects (in objects)
*/
int disk_cache_create(struct disk_cache* const disk_cache, const int object_size, const sector_t disk_size) {

assert(disk_cache);
assert(disk_size > 0);

disk_cache->object_size = object_size;
disk_cache->disk_size = disk_size;
disk_cache->objects_per_sector = bdev_get_queue(bdev)->hardsect_size / object_size;
disk_cache->start = start;
start += ((unsigned long)disk_size / (unsigned long)disk_cache->objects_per_sector) + 1;
return 1;
}
EXPORT_SYMBOL(disk_cache_create);

/*
* Returns an address where the requested object is found.
*/
void * get_object(const struct disk_cache* disk_cache, const sector_t object_number) {

int offset = 0;
sector_t sector = 0;
struct buffer_head *bh = NULL;

assert(disk_cache);
assert(object_number < disk_cache->disk_size);

/*
* Compute page number in which requested object resides.
*/
sector = disk_cache->start + (unsigned long)object_number / (unsigned long)disk_cache->objects_per_sector;
//assert(sector < disk_cache->disk_size);
//assert(sector < get_capacity(bdev->bd_disk));
offset = ((unsigned long)object_number % (unsigned long)disk_cache->objects_per_sector) * disk_cache->object_size;

bh = __bread(bdev, sector, bdev_get_queue(bdev)->hardsect_size);
//return bh->b_data + offset;
return NULL;
}
EXPORT_SYMBOL(get_object);

/*
* Puts given object back to the buffer cache. Flag 'modified' must be set if the object was modified.
*/
void put_object(const struct disk_cache* const disk_cache, const sector_t object_number, const int modified) {

struct buffer_head *bh = NULL;
sector_t sector = disk_cache->start + (unsigned long)object_number / (unsigned long)disk_cache->objects_per_sector;

bh = __bread(bdev, sector, bdev_get_queue(bdev)->hardsect_size);
//if(modified) {
// lock_buffer(bh);
// set_buffer_uptodate(bh);
// mark_buffer_dirty(bh);
// unlock_buffer(bh);
//}
brelse(bh);
/*
* an additional release, we didn't released it in get_object
*/
brelse(bh);
}
EXPORT_SYMBOL(put_object);

static void test(void) {

struct disk_cache dk;
int i;
void *p;

if(!disk_cache_create(&dk, 6, 100)) {
printk(KERN_ERR "create cache error\n");
return;
}
for(i = 0; i < 100; i++) {
get_object(&dk, i);
put_object(&dk, i, 0);
}
}

static int __init disk_cache_init(void) {

major_num = register_blkdev(major_num, "disk_cache");
if (major_num <= 0) {
printk(KERN_ERR "disk_cache: unable to get major number\n");
return -EINVAL;
}

if(!devname) {
printk(KERN_ERR "disk_cache: must supply a valid block device\n");
return -EINVAL;
}

bdev = open_bdev_excl(devname, 0, NULL);
if(IS_ERR(bdev)) {
printk(KERN_ERR "disk_cache: cannot open device %s.\n", devname);
return -EINVAL;
}

printk(KERN_INFO "disk_cache: using device %s\n", devname);
printk(KERN_INFO "disk_cache: %llu sectors\n", get_capacity(bdev->bd_disk));

test();

return 0;
}

static void __exit disk_cache_exit(void) {
close_bdev_excl(bdev);
}

module_init(disk_cache_init);
module_exit(disk_cache_exit);