Subject: Re: [PATCH 2/2] powerpc, kdump: Fix race in kdump shutdown

From: Michael Neuling <[email-masked]>
Date: Tue, 25 May 2010 09:53:44 +1000

In message <[email-masked]> you wrote
:
> 
> On May 24, 2010, at 2:23 PM, Kumar Gala wrote:
> 
> >=20
> > On May 14, 2010, at 12:40 AM, Michael Neuling wrote:
> >=20
> >> When we are crashing, the crashing/primary CPU IPIs the secondaries =
> to
> >> turn off IRQs, go into real mode and wait in kexec_wait.  While this
> >> is happening, the primary tears down all the MMU maps.  Unfortunately
> >> the primary doesn't check to make sure the secondaries have entered
> >> real mode before doing this.
> >>=20
> >> On PHYP machines, the secondaries can take a long time shutting down
> >> the IRQ controller as RTAS calls are need.  These RTAS calls need to
> >> be serialised which resilts in the secondaries contending in
> >> lock_rtas() and hence taking a long time to shut down.
> >>=20
> >> We've hit this on large POWER7 machines, where some secondaries are
> >> still waiting in lock_rtas(), when the primary tears down the HPTEs.
> >>=20
> >> This patch makes sure all secondaries are in real mode before the
> >> primary tears down the MMU.  It uses the new kexec_state entry in the
> >> paca.  It times out if the secondaries don't reach real mode after
> >> 10sec.
> >>=20
> >> Signed-off-by: Michael Neuling <[email-masked]>
> >> ---
> >>=20
> >> arch/powerpc/kernel/crash.c |   27 +++++++++++++++++++++++++++
> >> 1 file changed, 27 insertions(+)
> >>=20
> >> Index: linux-2.6-ozlabs/arch/powerpc/kernel/crash.c
> >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >> --- linux-2.6-ozlabs.orig/arch/powerpc/kernel/crash.c
> >> +++ linux-2.6-ozlabs/arch/powerpc/kernel/crash.c
> >> @@ -162,6 +162,32 @@ static void crash_kexec_prepare_cpus(int
> >> 	/* Leave the IPI callback set */
> >> }
> >>=20
> >> +/* wait for all the CPUs to hit real mode but timeout if they don't =
> come in */
> >> +static void crash_kexec_wait_realmode(int cpu)
> >> +{
> >> +	unsigned int msecs;
> >> +	int i;
> >> +
> >> +	msecs =3D 10000;
> >> +	for (i=3D0; i < NR_CPUS && msecs > 0; i++) {
> >> +		if (i =3D=3D cpu)
> >> +			continue;
> >> +
> >> +		while (paca[i].kexec_state < KEXEC_STATE_REAL_MODE) {
> >> +			barrier();
> >> +			if (!cpu_possible(i)) {
> >> +				break;
> >> +			}
> >> +			if (!cpu_online(i)) {
> >> +				break;
> >> +			}
> >> +			msecs--;
> >> +			mdelay(1);
> >> +		}
> >> +	}
> >> +	mb();
> >> +}
> >> +
> >> /*
> >> * This function will be called by secondary cpus or by kexec cpu
> >> * if soft-reset is activated to stop some CPUs.
> >> @@ -412,6 +438,7 @@ void default_machine_crash_shutdown(stru
> >> 	crash_kexec_prepare_cpus(crashing_cpu);
> >> 	cpu_set(crashing_cpu, cpus_in_crash);
> >> 	crash_kexec_stop_spus();
> >=20
> > should this be
> >=20
> > #ifdef CONFIG_PPC_STD_MMU
> >=20
> >> +	crash_kexec_wait_realmode(crashing_cpu);
> >=20
> > #endif
> 
> I'm going to make it CONFIG_PPC_STD_MMU_64 as part of a Kexec book-e =
> patch

Ok, thanks, I'll leave it up to you then

Mikey