[RFC PATCH] powerpc/64/kexec: fix race in kexec when XIVE is shutdown

["Cédric Le Goater" <clg@kaod.org>]

The kexec_state KEXEC_STATE_IRQS_OFF barrier is reached by all
secondary CPUs before the kexec_cpu_down() operation is called on
secondaries. This can raise conflicts and provoque errors in the XIVE
hcalls when XIVE is shutdowned with H_INT_RESET on the primary CPU.

To synchronize the kexec_cpu_down() operations and make sure the
secondaries have completed their task before the primary starts doing
the same, let's move the primary kexec_cpu_down() after the
KEXEC_STATE_REAL_MODE barrier.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---

 If this is a bad idea, there are alternate solutions :

 - introduce a new kexec_state and barrier
 - test for such error in all possible XIVE hcalls.

 But I tend to prefer the proposed one.

 Thanks,

 C.

 arch/powerpc/kernel/machine_kexec_64.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/machine_kexec_64.c b/arch/powerpc/kernel/machine_kexec_64.c
index 49d34d7271e7..212ecb8e829c 100644
--- a/arch/powerpc/kernel/machine_kexec_64.c
+++ b/arch/powerpc/kernel/machine_kexec_64.c
@@ -230,16 +230,16 @@ static void kexec_prepare_cpus(void)
 	/* we are sure every CPU has IRQs off at this point */
 	kexec_all_irq_disabled = 1;
 
-	/* after we tell the others to go down */
-	if (ppc_md.kexec_cpu_down)
-		ppc_md.kexec_cpu_down(0, 0);
-
 	/*
 	 * Before removing MMU mappings make sure all CPUs have entered real
 	 * mode:
 	 */
 	kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE);
 
+	/* after we tell the others to go down */
+	if (ppc_md.kexec_cpu_down)
+		ppc_md.kexec_cpu_down(0, 0);
+
 	put_cpu();
 }
 
-- 
2.13.6