[PATCH 3.19.y-ckt 091/146] powerpc/mce: fix off by one errors in mce event handling

From: Kamal Mostafa
Date: Wed Jun 17 2015 - 19:22:59 EST

3.19.8-ckt2 -stable review patch. If anyone has any objections, please let me know.


From: Daniel Axtens <dja@xxxxxxxxxx>

commit ffb2d78eca08a1451137583d4e435aecfd6af809 upstream.

Before 69111bac42f5 ("powerpc: Replace __get_cpu_var uses"), in
save_mce_event, index got the value of mce_nest_count, and
mce_nest_count was incremented *after* index was set.

However, that patch changed the behaviour so that mce_nest count was
incremented *before* setting index.

This causes an off-by-one error, as get_mce_event sets index as
mce_nest_count - 1 before reading mce_event. Thus get_mce_event reads
bogus data, causing warnings like
"Machine Check Exception, Unknown event version 0 !"
and breaking MCEs handling.

Restore the old behaviour and unbreak MCE handling by subtracting one
from the newly incremented value.

The same broken change occured in machine_check_queue_event (which set
a queue read by machine_check_process_queued_event). Fix that too,
unbreaking printing of MCE information.

Fixes: 69111bac42f5 ("powerpc: Replace __get_cpu_var uses")
CC: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxxxxxxx>
CC: Christoph Lameter <cl@xxxxxxxxx>
Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
arch/powerpc/kernel/mce.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index 15c99b6..b2eb468 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c
@@ -73,7 +73,7 @@ void save_mce_event(struct pt_regs *regs, long handled,
uint64_t nip, uint64_t addr)
uint64_t srr1;
- int index = __this_cpu_inc_return(mce_nest_count);
+ int index = __this_cpu_inc_return(mce_nest_count) - 1;
struct machine_check_event *mce = this_cpu_ptr(&mce_event[index]);

@@ -184,7 +184,7 @@ void machine_check_queue_event(void)
if (!get_mce_event(&evt, MCE_EVENT_RELEASE))

- index = __this_cpu_inc_return(mce_queue_count);
+ index = __this_cpu_inc_return(mce_queue_count) - 1;
/* If queue is full, just return for now. */
if (index >= MAX_MC_EVT) {

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