Patch "powerpc/mce: fix off by one errors in mce event handling" has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/mce: fix off by one errors in mce event handling

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-mce-fix-off-by-one-errors-in-mce-event-handling.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Axtens <[email protected]>
Date: Tue, 12 May 2015 13:23:59 +1000
Subject: powerpc/mce: fix off by one errors in mce event handling

From: Daniel Axtens <[email protected]>

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.

---
 arch/powerpc/kernel/mce.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -73,7 +73,7 @@ void save_mce_event(struct pt_regs *regs
 		    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))
 		return;
 
-	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) {
 		__this_cpu_dec(mce_queue_count);

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/powerpc-mce-fix-off-by-one-errors-in-mce-event-handling.patch