[PATCH v2] powerpc/mce: fix off by one errors in mce event handling

[Daniel Axtens <dja@axtens.net>]

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: stable@vger.kernel.org
CC: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
CC: Christoph Lameter <cl@linux.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>

---

The code is still super racy, but this at least unbreaks the common,
non-reentrant case for now until we figure out how to fix it properly.
The proper fix will likely be quite invasive so it might be worth
picking this up in stable rather than waiting for that?

mpe: the generated asm is below

0000000000000070 <.save_mce_event>:
  70:   e9 6d 00 30     ld      r11,48(r13)
  74:   3d 22 00 00     addis   r9,r2,0
  78:   39 29 00 00     addi    r9,r9,0
  7c:   7d 2a 4b 78     mr      r10,r9
  80:   39 29 00 08     addi    r9,r9,8
  84:   7d 8a 58 2e     lwzx    r12,r10,r11
  88:   39 8c 00 01     addi    r12,r12,1
  8c:   7d 8a 59 2e     stwx    r12,r10,r11
  90:   e9 0d 00 30     ld      r8,48(r13)
  94:   7d 4a 40 2e     lwzx    r10,r10,r8
  98:   39 4a ff ff     addi    r10,r10,-1
  9c:   2f 8a 00 63     cmpwi   cr7,r10,99

AIUI, we get the per-cpu area in 70, the addr of mce_nest_count itself
in 80, then load, incr, stor in 84-8c, then we get the address and
load again in 90-94, then subtract 1 to make the count sensible again,
then 9c is the conditional `if (index >= MAX_MC_EVT)'

I think that was what you expected?

Regards,
Daniel
---
 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))
 		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);
-- 
2.1.4