[PATCH] ARM: Temporary fix for broken arch reboot

[Russell King - ARM Linux]

On Fri, Nov 12, 2010 at 04:01:25PM +0530, Shilimkar, Santosh wrote:
> > -----Original Message-----
> > From: Russell King - ARM Linux [mailto:linux at arm.linux.org.uk]
> > Sent: Wednesday, November 10, 2010 9:16 PM
> > To: Shilimkar, Santosh
> > Cc: Catalin Marinas; linux-arm-kernel at lists.infradead.org; Gadiyar, Anand
> > Subject: Re: [PATCH] ARM: Temporary fix for broken arch reboot
> > 
> > On Wed, Nov 10, 2010 at 09:13:21PM +0530, Shilimkar, Santosh wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Catalin Marinas [mailto:catalin.marinas at arm.com]
> > > > Sent: Wednesday, November 10, 2010 7:35 PM
> > > > To: Russell King - ARM Linux
> > > > Cc: Shilimkar, Santosh; linux-arm-kernel at lists.infradead.org; Gadiyar,
> > > > Anand
> > > > Subject: Re: [PATCH] ARM: Temporary fix for broken arch reboot
> > > >
> > > > The printing only happens for SYSTEM_BOOTING or SYSTEM_RUNNING. I
> > > > suspect in this case we have SYSTEM_RESTARTING and the condition in
> > > > ipi_cpu_stop() is false, therefore no printing. It may be worth
> > putting
> > > > some printks outside the 'if' to see whether the secondary CPUs get
> > > > there.
> > > >
> > > While doing some experiments on this issue, one interesting
> > > observation I made. Looks like there is race between two
> > > Cores which makes system behave badly in reboot path.
> > >
> > > Just adding a delay in the ipi_cpu_stop() makes the reboot work
> > > as well
> > 
> > That could just be that it's taking longer than 500us for the reboot CPU
> > to get through setup_mm_for_reboot().
> 
> It not seems to be the case otherwise below patch should have worked.
> Right ? Did I miss your point ?

Yes you did.  Maybe all this code completes before your 500us in the
secondary CPUs complete.  So the delay in the secondary CPU path proves
nothing.

A delay in the reboot CPU path ensures that the other CPUs have time to
react to the stop IPI, so we know what state the system is in.