On Tue, Apr 26, 2022 at 02:12:22PM -0400, Demi Marie Obenour wrote:
> Right now, opening block devices in a race-free way is incredibly hard.
> The only reasonable approach I know of is sd_device_new_from_path() +
> sd_device_open(), and is only available in systemd git main. It also
> requires waiting on systemd-udev to have processed udev rules, which can
> be a bottleneck. There are better approaches in various special cases,
> such as using device-mapper ioctls to check that the device one has
> opened still has the name and/or UUID one expects. However, none of
> them works for a plain call to open(2).
Why do you call open(2) on a block device?
> A much better approach would be for udev to point its symlinks at
> "/dev/disk/by-diskseq/$DISKSEQ" for non-partition disk devices, or at
> "/dev/disk/by-diskseq/${DISKSEQ}p${PARTITION}" for partitions.
You can do that today with udev rules, right?
> A
> filesystem would then be mounted at "/dev/disk/by-diskseq" that provides
> for race-free opening of these paths.
How would it be any less race-free than just open("/dev/sda1") is?
> This could be implemented in
> userspace using FUSE, either with difficulty using the current kernel
> API, or easily and efficiently using a new kernel API for opening a
> block device by diskseq + partition. However, I think this should be
> handled by the Linux kernel itself.
>
> What would be necessary to get this into the kernel?
Get what exactly? I don't see anything the kernel needs to do here
specifically. Normally block devices are accessed using mount(2), not
open(2). Do you want a new mount(2)-type api?
thanks,
greg k-h
