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Gmane
From: Al Viro <viro <at> ftp.linux.org.uk>
Subject: Re: Race between "mount" uevent and /proc/mounts?
Newsgroups: gmane.linux.hotplug.devel
Date: Wednesday 26th October 2005 19:28:59 UTC (over 12 years ago)
On Wed, Oct 26, 2005 at 04:34:17PM +0200, Kay Sievers wrote:
> > Semantics for events depends on which objects you are interested in.
> > Existing ones do not match _any_ of the real objects and I have no
> > idea what exactly had been intended for them.  I've asked gregkh, but
> > he didn't remember that either.  Apparently they are used by different
> > people as (bad) approximations to different things.  Which doesn't work
> > well.  And until somebody cares to describe what exactly are they
trying
> > to watch the situation obviously won't improve.
> 
> They are actually events for claim/release of a block device. As uevents
> are bound to kobjects we needed to send these events from an existing
device
> which is the blockdev itself.
> 
> Sure, the event itself, has nothing to do with a filesystem. The names
are
> like this for historical reasons and "CLAIM/RELEASE" may be less
confusing.
> The events are used as a trigger to rescan /proc/mounts instead of
polling
> it constantly.

But that makes no sense.  /proc/*/mounts changes when mount tree changes.
Which is obviously not an event happening to block devices.  Moreover,
changes of mount tree may involve no changes in the set of active
filesystems
or be separated in time from such changes by arbitrary intervals.

Looks like seriously wrong assumptions in userland code working with these
events...  _IF_ you want to keep track of /proc/*/mounts changes, the
obvious
solution would be to implement ->poll() for them.  However, if you are
really interested in block devices, keep in mind that
	* getting them claimed happens before your event is generated
	* eventually the filesystem claiming them becomes active (or doesn't,
if mount fails)
	* eventually an active fs may (or may not) become visible in mount
tree.
	* not every umount leads to deactivation
	* deactivation can happen long after the fs is no longer present in
mount tree
	* fs may become visible in mount tree again without being deactivated
and activated again - (mount /dev/foo /mnt; exec poll() rather than wanking with
events).  If you want something more complex, you might or might not be
SOL, depending on what you are trying to achieve.
 
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