Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

[Jann Horn]

On Thu, Jul 12, 2018 at 3:54 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Theodore Y. Ts'o <tytso@xxxxxxx> wrote:
>
> > So maybe the answer is that you open /dev/sda1 and /dev/sda2 and then
> > pass the file descriptors to the fsopen object?  We can require that
> > the fd's be opened with O_RDWR and O_EXCL, which has the benefit where
> > if you have multiple block devices, you know *which* block device had
> > a problem with being grabbed for an exclusive open.
>
> Would that mean then that doing:
>
>         mount /dev/sda3 /a
>         mount /dev/sda3 /b
>
> would then fail on the second command because /dev/sda3 is already open
> exclusively?

Not exactly. mount_bdev() uses FMODE_EXCL, which locks out parallel
usage *with a different filesystem type*. This is the effect:

# strace -e trace=mount mount -t vfat /dev/loop0 mount
mount("/dev/loop0", "/home/jannh/tmp/x/mount", "vfat", MS_MGC_VAL, NULL) = 0
+++ exited with 0 +++
# strace -e trace=mount mount -t ext4 /dev/loop0 mount
mount("/dev/loop0", "/home/jannh/tmp/x/mount", "ext4", MS_MGC_VAL,
NULL) = -1 EBUSY (Device or resource busy)
mount: /home/jannh/tmp/x/mount: /dev/loop0 already mounted on
/home/jannh/tmp/x/mount.
+++ exited with 32 +++

I don't really understand why it's not more strict though...