Forcible removal of modules

Forcible removal of modules

[Thomas Hood]

Hi.  Here's my question, with a little introduction.

Sometimes modules need to be reloaded in order
to cause some sort of reinitialization (of the
driver or of the hardware) to occur.
Sometimes this has to be done every time a machine
is suspended.  E.g., some sound driver modules
need to be reloaded after an APM suspend because
the sound chip forgets its configuration during the
suspend.  Obviously, one would like to be able to
automate the process of unloading and reloading the
modules by putting some commands in the apmd_proxy
script. Sometimes, though, "rmmod themodulename" fails
because some process is holding open a device handled
by the device driver module in question.

The solution to this is to do a "fuser" on the device
nodes associated with the device driver and kill all
the processes reported to be using those nodes.  
However, this is easier said than done because of one
problem: while you are killing a batch of processes
that are using the device node, other processes may
be opening that device node!  What is needed is a way
of preventing processes from opening a given device node.  

Now, one way of doing this is to change the device
permissions on the node to 000.  Unfortunately this does
not work well with devfs under the circumstances we have
in mind here: because once the permissions are changed
the device driver is removed; then devfs stores "000"
as the permissions that are to be used for that device
node when it is created again.

My question is: Is there some better way of blocking 
all open() calls to a particular device driver while
processes using it are being killed off?

Thomas Hood





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Re: Forcible removal of modules

[Keith Owens]

On Tue, 6 Mar 2001 14:17:28 -0800 (PST), 
Thomas Hood <thood@excite.com> wrote:
>My question is: Is there some better way of blocking 
>all open() calls to a particular device driver while
>processes using it are being killed off?

Not yet.  There have been some off list discussions about redoing the
module load/unload process to avoid races, as part of that we will get
forced module unregister followed by unload when the use count goes to
zero.  Probably 2.5 changes.

Re: Forcible removal of modules

[John Fremlin]

 Thomas Hood <thood@excite.com> writes:

> Sometimes modules need to be reloaded in order
> to cause some sort of reinitialization (of the
> driver or of the hardware) to occur.

Why not set up the device driver to handle PM events itself. See
Documentation/pm.txt under Driver Interface.

I have a race free version of pm_send_all if you want it.

[...]

-- 

	http://www.penguinpowered.com/~vii

Re: Forcible removal of modules

[Jeff Garzik]

John Fremlin wrote:
> Why not set up the device driver to handle PM events itself. See
> Documentation/pm.txt under Driver Interface.

For PCI drivers, you implement the ::suspend and ::remove hooks.

> I have a race free version of pm_send_all if you want it.

Is this the same thing that is in 2.4.3-pre3?

-- 
Jeff Garzik       | "You see, in this world there's two kinds of
Building 1024     |  people, my friend: Those with loaded guns
MandrakeSoft      |  and those who dig. You dig."  --Blondie

Racing power management

[John Fremlin]

 Jeff Garzik <jgarzik@mandrakesoft.com> writes:

> John Fremlin wrote:
> > Why not set up the device driver to handle PM events itself. See
> > Documentation/pm.txt under Driver Interface.
> 
> For PCI drivers, you implement the ::suspend and ::remove hooks.
> 
> > I have a race free version of pm_send_all if you want it.
> 
> Is this the same thing that is in 2.4.3-pre3?

Aarrgh. Looks like Alan Cox got his version in kernel first. (I did
write mine before.)

If I am not mistaken there is one (hypothetical) race still remaining
in Alan's version. Last time I checked the only code doing pm_send_all
was in the i386 APM driver (and so of course there is no chance of any
race at all even with the old version, if I understand correctly).

But suppose there were another pm_send_all caller. APM would decide to
user suspend and call pm_send_all asking for a SUSPEND to check it was
allowed to. While this is happening some clueless loser decides to
pm_send_all RESUME for whatever reason. This loser has to wait until
the APM pm_send_all finishes, but hypothetically and if I am not
mistaken, his pm_send_all RESUME could get in just after the APM
pm_send_all finished and just before APM made the physical BIOS call
to suspend the machine. This would screw stuff up of course.

You may say, this is rather improbable if not impossible, but it does
suggest that it would be a good idea to have locking wrapping
pm_send_all and the hardware machine suspend request. Cue: John's
pmpolicy patch.

Unfortunately, my patch adds another thing as well: /sbin/powermanager
so it got (is getting) trampled on by a lot of people who didn't like
that idea. If anybody wants the race free PM by itself, I can factor
out that bit.

-- 

	http://www.penguinpowered.com/~vii

Re: Forcible removal of modules

[Alan Cox]

> For PCI drivers, you implement the ::suspend and ::remove hooks.
> > I have a race free version of pm_send_all if you want it.
> Is this the same thing that is in 2.4.3-pre3?

Mine is race free for the basics, his is a far far more elegant solution to the
whole problem space. It might be 2.5 stuff but its definitely a good idea