[systemd-devel] How to stop systemd-udevd reading a device after dd

[Lennart Poettering]

On Fr, 13.10.17 01:01, Akira Hayakawa (ruby.wktk at gmail.com) wrote:

> I have a device /dev/sdb1 and let's trace the block request by blktrace
> 
> $ sudo blktrace -d /dev/sdb1
> 
> When I write 4KB using dd
> $ sudo dd if=/dev/zero of=/dev/sdb1 oflag=direct bs=4k count=1
> 
> The block trace (after blkparsed) is write request as expected
>   8,17   2        2     0.000003171  5930  Q  WS 2048 + 8 [dd]
> 
> followed by a unexpected read from systemd-udevd
>   8,17   7        2     0.001755563  5931  Q   R 2048 + 8 [systemd-udevd]
> 
> My first question is what is: this read request?
> 
> And I want to stop the read request because it makes it difficult to test kernel code.
> So the second question is: how can I stop the read request?

If you want exclusive access to a block device and don't want udev to
step in, then simply take a BSD file lock on it (i.e. flock(2)), and
udev won't probe it.

This is not particularly well documented (one could say: not at all),
but it's the official way really, and very UNIXish I'd claim.

Lennart

-- 
Lennart Poettering, Red Hat