Re: [Qemu-devel] [PATCH RFC] file-posix: make lock_fd read-only

[Kevin Wolf <kwolf@redhat.com>]

[ Cc: Fam ]

Am 10.10.2017 um 15:42 hat Vladimir Sementsov-Ogievskiy geschrieben:
> We do not reopen lock_fd on bdrv_reopen which leads to problems on
> reopen image RO. So, lets make lock_fd be always RO.
> This is correct, because qemu_lock_fd always called with exclusive=false
> on lock_fd.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all!
> 
> We've faced the following problem with our shared-storage migration
> scheme. We make an external snapshot and need base image to be reopened
> RO. However, bdrv_reopen reopens only .fd of BDRVRawState but not
> .lock_fd. So, .lock_fd is left opened RW and this breaks the whole
> thing.
> 
> The simple fix is here: let's just open lock_fd as RO always. This
> looks fine for current code, as we never try to set write locks
> (qemu_lock_fd always called with exclusive=false).
> 
> However it will not work if we are going to use write locks.

I was sure that we had discussed this during review, so I just went back
and checked. Indeed, Fam originally had an unconditional O_RDONLY in
some version of the image locking patches, but I actually found a
potential problem with that back then:

> Note that with /dev/fdset there can be cases where we can open a file
> O_RDWR, but not O_RDONLY. Should we better just use the same flags as
> for the s->fd?
https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg05107.html

However, I'm now wondering whether we really still need a separate
s->lock_fd or whether we can just use the normal image fd for this. If I
understood the old threads correctly, the original reason for it was
that during bdrv_reopen(), we couldn't safely migrate exclusive locks
from the old fd to the new one. But as we aren't using exclusive locks
any more, this shouldn't be a problem today.

Fam, are there more reasons why we need a separate lock_fd?

Kevin