Re: [PATCH] Btrfs-progs: fix race condition between btrfs and udev

[Anand Jain <anand.jain@oracle.com>]

Wang,

> Futher, a easy way to trigger this problem is by running the following
> c codes repeatedly:
>
>   int main(int argc, char **argv)
>   {
>   	/* pass a btrfs block device */
>   	int fd = open(argv[1], O_RDWR | O_EXCL);
> 	if (fd < 0) {
> 		perror("fail to open: %s", strerror(errno));
> 		exit(1);
> 	}
> 	close(fd);
> 	return 0;
>   }
>
> So the problem is RW opening would trigger udev event which will
> call btrfs_scan_one_device().


> In btrfs_scan_one_device(), it
> would open the block device with EXCL flag..meanwhile if another
> program try to open that device with O_EXCL, it would fail with
> EBUSY.

  Expected. But do we need O_EXCL in kernel:btrfs_scan_one_device()
  at all ?

> This happen seldomly in the real world, but if we use loop device
> for test, we may hit this annoying problem.

> A walkaround way to solve this problem is to wait kernel scanning
> finished and then try it again.

  I agree this happens seldom in production. But I don't agree on
  something to fix as workaround.

  Just sent out the patch:
      btrfs: looping mkfs.btrfs -f <dev> may fail with EBUSY

  which I believe can be a final fix. Your review / tests appreciated.


Thanks, Anand