restart callback not supposed to sleep?

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Aug 20, 2012 at 10:57:34AM +0200, Lothar Wa?mann wrote:
> Hi,
> 
> Uwe Kleine-K?nig writes:
> > Hello,
> > 
> > while triggering a reboot via sysrq on an ARM machine I saw a "BUG:
> > sleeping function called from invalid context" message (details below).
> > The reason is that the sysrq handler is calling the restart hook in irq
> > context. So what should be done about that? The obvious possiblities
> > are:
> > 
> >  a) ignore the problem as it won't result in problems (and maybe somehow
> >     silence the warnign); or
> >  b) make the restart hook atomic; or
> >  c) assert sysrq not calling the hook in atomic mode
> > 
> > What do you think?
> > 
> This has been discussed earlier (with your participation) with 'a)'
> being the conclusion.
> http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/033877.html

Except if you look at the backtrace, it seems that it fails to restart.
There are two attempts there at making it restart so the first could
not have succeeded.