[systemd-devel] Inhibiting plug and play

[Phillip Susi]

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On 6/18/2013 2:03 PM, David Zeuthen wrote:
> When I was younger I used to think things like this was a good
> idea and, in fact, did a lot of work to add complex interfaces for
> this in the various components you mention. These interfaces didn't
> really work well, someone would always complain that this or that
> edge-case didn't work. Or some other desktop environment ended up
> not using the interfaces. Or some kernel hacker running twm (with
> "carefully" selected bits of GNOME or KDE to get automounting) ran
> into problems. It was awful. Just awful.

I can't really extract any meaning from this without knowledge of what
was tried and what problems it caused.  I also don't see why it can't
be something as simple as opening the device with O_EXCL.

> What _did_ turn out to work really well - and what GNOME is using 
> today and have been for the last couple of years - is that the 
> should_automount flag [1] is set only if, and only if, the device
> the volume is on, has been added within the last five seconds [2].
> It's incredibly simple (and low-tech). And judging from bug
> reports, it works really well.

I don't follow.  You mean udisks delays auto mounting by 5 seconds?
That's not going to help if, for instance, you use gparted to move a
partition to the right.  It first enlarges the partition, which
generates a remove/add event, then starts moving data.  5 seconds
later udisks tries to mount the partition, which very well may succeed
with horrible consequences.

The problem also goes beyond udisks and auto mounting, which is why I
say it really needs done either at the udev or kernel level.

For instance, a udev script may identify the new volume as part of a
raid ( leftover metadata ) and try to attach mdadm to it, at the same
time you're running mkfs.  I'm also pretty sure that I have seen the
mdadm udev script race with mdadm itself while you are trying to
create a new raid volume.


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