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

[Kumar Gala]

On May 24, 2010, at 2:23 PM, Kumar Gala wrote:

> 
> On May 14, 2010, at 12:40 AM, Michael Neuling wrote:
> 
>> 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.
>> 
>> 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.
>> 
>> We've hit this on large POWER7 machines, where some secondaries are
>> still waiting in lock_rtas(), when the primary tears down the HPTEs.
>> 
>> 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.
>> 
>> Signed-off-by: Michael Neuling <mikey at neuling.org>
>> ---
>> 
>> arch/powerpc/kernel/crash.c |   27 +++++++++++++++++++++++++++
>> 1 file changed, 27 insertions(+)
>> 
>> Index: linux-2.6-ozlabs/arch/powerpc/kernel/crash.c
>> ===================================================================
>> --- 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 */
>> }
>> 
>> +/* 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 = 10000;
>> +	for (i=0; i < NR_CPUS && msecs > 0; i++) {
>> +		if (i == 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();
> 
> should this be
> 
> #ifdef CONFIG_PPC_STD_MMU
> 
>> +	crash_kexec_wait_realmode(crashing_cpu);
> 
> #endif

I'm going to make it CONFIG_PPC_STD_MMU_64 as part of a Kexec book-e patch

- k