Potential concurrency bug in ide-disk.c ?

Tushar Adeshara
2005-09-02T11:38:33Z

Hi,
The way file ide-disk.c handles usage count, it seems to me that its
concurrency bug.
In open method and release, it uses code as follows

static int idedisk_open(struct inode *inode, struct file *filp)
{
ide_drive_t *drive = inode->i_bdev->bd_disk->private_data;
drive->usage++;
if (drive->removable && drive->usage == 1) {
ide_task_t args;
memset(&args, 0, sizeof(ide_task_t));
args.tfRegister[IDE_COMMAND_OFFSET] = WIN_DOORLOCK;
args.command_type = IDE_DRIVE_TASK_NO_DATA;
args.handler = &task_no_data_intr;
check_disk_change(inode->i_bdev);
/*
* Ignore the return code from door_lock,
* since the open() has already succeeded,
* and the door_lock is irrelevant at this point.
*/
if (drive->doorlocking && ide_raw_taskfile(drive, &args, NULL))
drive->doorlocking = 0;
}
return 0;
}

Here, if drive->usage=0 initially and two process concurrently executes
drive->usage++, then drive->usage will become 2. Both of them will
think that drive is already initialized. Something similar can happen
in case of release.
I think a semaphore need to be added in
ide_drive_t structure and method should be modified as

static int idedisk_open(struct inode *inode, struct file *filp)
{
ide_drive_t *drive = inode->i_bdev->bd_disk->private_data;
if(down_interruptible(&drive->sem)){
/*error handling code*/
}
drive->usage++;
if (drive->removable && drive->usage == 1) {
ide_task_t args;
memset(&args, 0, sizeof(ide_task_t));
args.tfRegister[IDE_COMMAND_OFFSET] = WIN_DOORLOCK;
args.command_type = IDE_DRIVE_TASK_NO_DATA;
args.handler = &task_no_data_intr;
check_disk_change(inode->i_bdev);
/*
* Ignore the return code from door_lock,
* since the open() has already succeeded,
* and the door_lock is irrelevant at this point.
*/
if (drive->doorlocking && ide_raw_taskfile(drive, &args, NULL))
drive->doorlocking = 0;
}
up(&drive->sem);
return 0;
}
Similar modifications are also required in release.

Please let me know if there is anything wrong in above code. Also let
me know to whom I should offer patches for this.

--
Regards,
Tushar
--------------------
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