Re: [PATCH] In-kernel module loader 1/7

[Daniel Phillips]

On Thursday 17 October 2002 00:48, Rusty Russell wrote:
> > On Wednesday 16 October 2002 08:11, Rusty Russell wrote:
> > > It needs to be turned off when dealing with any interface which might
> > > be used by one of the hard modules.  Which is pretty bad.
> > 
> > As far as I can see, preemption only has to be disabled during the 
> > synchronize_kernel phase of unloading that one module, and this requirement 
> > is inherited neither by dependant or depending modules.
> 
> No, someone could already have been preempted before you start
> synchronize_kernel().

I don't get that.  The sequence is:

  - turn off preemption
  - unhook call points
  - synchronize_kernel
  - ...

which doesn't leave any preemption hole that I can see, so I can't comment
on a couple of the other points until you clear that one up.

> > > Now, there remains a subtle problem with the try_inc_mod_count
> > > approach in general.  It is the "spurious failure" problem, where
> > > eg. a notifier cannot inc the module count, and so does not call the
> > > registered notifier, but the module is still being initialized *OR* is
> > > in the middle of an attempt to remove the module (which fails, and the
> > > module is restored to "life").
> > 
> > For pure counting-style modules, it's easy to avoid this problem: the module
> > is placed in the can't-increment state if and only if the current count is 
> > zero, and from that point on we know the unload will either succeed or fail 
> > with an error.
> 
> Still a race between the zero check and the can't-increment state
> setting.

But that one is easy: the zero check just takes the same spinlock as 
TRY_INC_MOD_COUNT, then sets can't-increment only in the case the count
is zero, considerably simpler than:

> This is what my current code does: rmmod itself checks (if
> /proc/modules available), then the kernel sets the module to
> can't-increment, then checks again.  If the non-blocking flag is set,
> it then re-animates the module and fails, otherwise it waits.

and leaves no window for spurious failure.  The still-initializing case is
also easy, e.g., a filesystem module simply doesn't call register_filesystem
until it's completely ready to service calls, so nobody is able to do
TRY_INC_MOD_COUNT.

> BTW, current patchset (2.5.43):

Thanks, I'll read them all on the 21st ;-)  The other thing I need to read
closely is Roman's strategy for changing the module format, and the weird
linker connections.

> ...The second is the "die-mother-fucker-die"
> version, which taints the kernel and just removes the damn thing.  For
> most people, this is better than a reboot, and will usually "work".

Is there a case where removing a module would actually help?  What is
the user going to do next, try to reinsert the same module?

> http://www.kernel.org/pub/linux/kernel/people/rusty/patches/Module/force-unload.patch.gz
> [ Doesn't apply currently, needs updating ]

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-- 
Daniel