Re: [RFC] Suspicious bug in module refcounting

[Dan Williams]

On Tue, Feb 3, 2009 at 8:48 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
> On Wednesday 04 February 2009 00:17:21 Karsten Keil wrote:
>> The refcount is a per CPU atomic variable, module_refcount() simple add
>> in a fully unprotected loop (not disabled irqs, not protected against
>> scheduling) all per cpu values.
>
> Hi Karsten,
>
>   Yes, the BUG_ON() is overly aggressive.  And I really hate __module_get,
> and it looks like most of the callers are completely bogus.  The watchdog
> drivers use it to nail themselves in place in their open routines: this is
> OK, if a bit weird.
>
>   We should only use __module_get() when you *can't handle* failure;
> otherwise you should accept that the admin did rmmod --wait and don't use the
> module any further.
>
>  dmaengine.c seems to be taking liberties like this.  AFAICT it can error
> out, so why not just try_module_get() always?

Currently there is no feedback loop for clients calling
dmaengine_get().  It simply means "I want to do offload, pin any
offload resources you may see, and don't let the resource leave until
dmaengine_ref_count == 0".  Even if we always called try_module_get()
we would still need to wait until dmaengine_ref_count reached zero to
be sure no transactions are in flight, effectively ignoring module_get
failures.

However, dma-driver module removal is still in the central control of
the administrator as downing all network interfaces and unloading the
async_tx api (i.e. raid456) will kill all dmaengine references.  We
just have the caveat highlighted below:

modprobe raid456
ifup eth0
rmmod --wait ioat_dma &
ifup eth1
modprobe -r raid456
ifdown eth0  <-- module removal succeeds here in a perfect world
ifdown eth1 <-- module removal currently succeeds here

Regards,
Dan
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