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

[Kumar Gala]

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

- k