Re: [PATCH 2/2] powerpc/book3s: mce: Use add_taint_no_warn() in machine_check_early().

On 04/17/2017 04:09 PM, Daniel Axtens wrote:

Hi Mahesh,

quoted

Fixes: 27ea2c420cad powerpc: Set the correct kernel taint on machine check errors.

I notice this Fixes a commit I introduced. Please could you cc me when
you do this? I am likely to miss it otherwise, especially since I have
now left IBM.

Sure will do. :-)

Being cced allows me to provide an Ack or a review. And getting feedback
on my changes is very helpful in becoming a better programmer.

In this case, as per Michael's comment, why don't we just move the
add_taint from machine_check_early to
machine_check_process_queued_event - the other side of the work queue.

Yes. That is what my plan is. Also, that is not the only place.
add_taint() need to be called from machine_check_exception() as well. So
it will be called from two places.

Thanks,
-Mahesh.

The work queue system is supposed to provide us with a safe place to do
printing, etc., so it's an appropriate place. Also, we already do
machine_check_print_event_info there, and adding the taint doesn't need
to be done synchronously.

Regards,
Daniel

Mahesh J Salgaonkar [off-list ref] writes:

quoted

From: Mahesh Salgaonkar <redacted>

machine_check_early() gets called in real mode. The very first time when
add_taint() is called, it prints a warning which ends up calling opal
call (that uses OPAL_CALL wrapper) for writing it to console. If we get a
very first machine check while we are in opal we are doomed. OPAL_CALL
overwrites the PACASAVEDMSR in r13 and in this case when we are done with
MCE handling the original opal call will use this new MSR on it's way
back to opal_return. This usually leads unexpected behaviour or kernel
to panic. Instead use the add_taint_no_warn() that does not call printk.

This is broken with current FW level. We got lucky so far for not getting
very first MCE hit while in OPAL. But easily reproducible on Mambo.
This should go to stable as well alongwith patch 1/2.

Signed-off-by: Mahesh Salgaonkar <redacted>
---
 arch/powerpc/kernel/traps.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 62b587f..4a048dc 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c

@@ -306,7 +306,7 @@ long machine_check_early(struct pt_regs *regs)
 
 	__this_cpu_inc(irq_stat.mce_exceptions);
 
-	add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
+	add_taint_no_warn(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
 
 	/*
 	 * See if platform is capable of handling machine check. (e.g. PowerNV