Unmatched semaphore usage in fs/super.c

Unmatched semaphore usage in fs/super.c

[Peter Teoh]

Please pardon me if wrong.   In fs/super.c, I saw a down_read()
without a matching (when sb->s_root is non-zero) up_read() as shown
below:

/**
 *      get_super - get the superblock of a device
 *      @bdev: device to get the superblock for
 *
 *      Scans the superblock list and finds the superblock of the file system
 *      mounted on the device given. %NULL is returned if no match is found.
 */

struct super_block * get_super(struct block_device *bdev)
{
       struct super_block *sb;

       if (!bdev)
               return NULL;

       spin_lock(&sb_lock);
rescan:
       list_for_each_entry(sb, &super_blocks, s_list) {
               if (sb->s_bdev == bdev) {
                       sb->s_count++;
                       spin_unlock(&sb_lock);
                       down_read(&sb->s_umount);
                       if (sb->s_root)
                               return sb;
                       up_read(&sb->s_umount);
                       /* restart only when sb is no longer on the list */
                       spin_lock(&sb_lock);
                       if (__put_super_and_need_restart(sb))
                               goto rescan;
               }
       }
       spin_unlock(&sb_lock);
       return NULL;
}

Neither did the get_super() caller remember and correct the semaphore
either.   Is this a problem?

Re: Unmatched semaphore usage in fs/super.c

[Al Viro]

On Sun, Mar 02, 2008 at 03:10:35PM +0800, Peter Teoh wrote:
> Please pardon me if wrong.   In fs/super.c, I saw a down_read()
> without a matching (when sb->s_root is non-zero) up_read() as shown
> below:

Correct.  get_super() and user_get_super() return locked superblock.

> Neither did the get_super() caller remember and correct the semaphore
> either.   Is this a problem?

It would be, if it would be true.  Take a look at drop_super().