powerpc/kdump: Fix race in kdump shutdown

powerpc/kdump: Fix race in kdump shutdown

Commit: 60adec6226bbcf061d4c2d10944fced209d1847d upstream

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.

---
 arch/powerpc/kernel/crash.c |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)

index b779818..706b622 100644
@@ -153,6 +153,7 @@ static void crash_kexec_prepare_cpus(int cpu)

		while (cpus_weight(cpus_in_crash) < ncpus)
			cpu_relax();
	}

+
/*
* Make sure all CPUs are entered via soft-reset if the kdump is
* invoked using soft-reset.
@@ -162,6 +163,32 @@ static void crash_kexec_prepare_cpus(int cpu)

	/* 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.
@@ -419,6 +446,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)

	crash_kexec_prepare_cpus(crashing_cpu);
	cpu_set(crashing_cpu, cpus_in_crash);
	crash_kexec_stop_spus();
+	crash_kexec_wait_realmode(crashing_cpu);
	if (ppc_md.kexec_cpu_down)
		ppc_md.kexec_cpu_down(1, 0);

}

Please don't indent these, I have to unindent them for every patch I apply from you, which is a pain.