[PATCH] x86_64: Fix mce_log

The attempt to fixup the lockless mce log buffer introduced an infinite loop
when trying to find a free entry.

And:

Using rcu_dereference() to load mcelog.next doesn't seem to be sufficient
enough to ensure that mcelog.next is loaded each time around the loop in
mce_log().  Instead, use an explicit rmb() to ensure that the compiler gets it
right.

AK: turned the smp_wmbs into true wmbs to make sure they are not
reordered by the compiler on UP.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/arch/x86_64/kernel/mce.c b/arch/x86_64/kernel/mce.c
index 08203b07f4bd..69541db5ff2c 100644
--- a/arch/x86_64/kernel/mce.c
+++ b/arch/x86_64/kernel/mce.c
@@ -54,9 +54,12 @@ void mce_log(struct mce *mce)
 {
         unsigned next, entry;
         mce->finished = 0;
-        smp_wmb();
+        wmb();
         for (;;) {
                 entry = rcu_dereference(mcelog.next);
+                /* The rmb forces the compiler to reload next in each
+                    iteration */
+                rmb();
                 for (;;) {
                         /* When the buffer fills up discard new entries. Assume
                            that the earlier errors are the more interesting. */
@@ -69,6 +72,7 @@ void mce_log(struct mce *mce)
                                 entry++;
                                 continue;
                         }
+                        break;
                 }
                 smp_rmb();
                 next = entry + 1;
@@ -76,9 +80,9 @@ void mce_log(struct mce *mce)
                         break;
         }
         memcpy(mcelog.entry + entry, mce, sizeof(struct mce));
-        smp_wmb();
+        wmb();
         mcelog.entry[entry].finished = 1;
-        smp_wmb();
+        wmb();
 
         if (!test_and_set_bit(0, &console_logged))
                 notify_user = 1;