Re: [PATCH] powerpc/book3s: Fix MCE console messages for unrecoverable MCE.

On 08/04/2016 03:27 PM, Michael Ellerman wrote:

Mahesh J Salgaonkar [off-list ref] writes:

quoted

From: Mahesh Salgaonkar <redacted>

When machine check occurs with MSR(RI=0), it means MC interrupt is
unrecoverable and kernel goes down to panic path. But the console
message still shows it as recovered. This patch fixes the MCE console
messages.

Signed-off-by: Mahesh Salgaonkar <redacted>
---
 arch/powerpc/kernel/mce.c             |    3 ++-
 arch/powerpc/platforms/powernv/opal.c |    2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index ef267fd..5e7ece0 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c

@@ -92,7 +92,8 @@ void save_mce_event(struct pt_regs *regs, long handled,
 	mce->in_use = 1;
 
 	mce->initiator = MCE_INITIATOR_CPU;
-	if (handled)
+	/* Mark it recovered if we have handled it and MSR(RI=1). */
+	if (handled && (regs->msr & MSR_RI))
 		mce->disposition = MCE_DISPOSITION_RECOVERED;

This seems like it has bigger implications than just changing the
printk output? We're now (correctly) marking any MC where RI=0 as
unrecoverable.

Or is the only place that uses this the code below which *also* checks
MSR_RI?

We would always check MSR_RI at code below and panic correctly. It was
just that we were always printing it as recovered and then panic.

quoted

diff --git a/arch/powerpc/platforms/powernv/opal.c b/arch/powerpc/platforms/powernv/opal.c
index 5385434..8154171 100644
--- a/arch/powerpc/platforms/powernv/opal.c
+++ b/arch/powerpc/platforms/powernv/opal.c

@@ -401,6 +401,8 @@ static int opal_recover_mce(struct pt_regs *regs,
 
 	if (!(regs->msr & MSR_RI)) {
 		/* If MSR_RI isn't set, we cannot recover */

Why do we check MSR_RI again here? Shouldn't we just be looking at the evt->disposition?

When MSR_RI=0, where SRR0/SRR1 registers values have been thrashed,
kernel can not continue reliably if we return from interrupt. It should
definitely go down to panic path. Hence we check for RI=0 and return 0.
Where as, if MSR_RI=1 and disposition is "unrecovered", we can minimise
the damage to user process if this MCE was hit in user space context.

The print is just to tell that the kernel panic'ed because MCE occured
during a rare window where MSR RI bit was set to zero(0) and not that
handler could not fix the error.

quoted

+		printk(KERN_ERR "Machine check interrupt unrecoverable:"
+				" MSR(RI=0)\n");

Are we sure it's safe to call printk() there?

Yes, we had just printed MCE event info before we came here.

Please don't split the message across lines, and use pr_err() like the
rest of the code in this file. So it would be:

		pr_err("Machine check interrupt unrecoverable: MSR(RI=0)\n");

Sure. Will make the change.

quoted

 		recovered = 0;
 	} else if (evt->disposition == MCE_DISPOSITION_RECOVERED) {
 		/* Platform corrected itself */

cheers