[PATCH] x86-64: Make lockless machine check record passing a bit more robust.

One machine is constantly throwing NMI watchdog timeouts in mce_log

This was one attempt to fix it.

(AK: this doesn't actually fix the bug I'm seeing unfortunately, probably
drop.  I don't like it that the reader can spin forever now waiting
for a writer)

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 11 deletions

diff --git a/arch/x86_64/kernel/mce.c b/arch/x86_64/kernel/mce.c
index 8aa56736cde3..87ea8fdd43fd 100644
--- a/arch/x86_64/kernel/mce.c
+++ b/arch/x86_64/kernel/mce.c
@@ -56,15 +56,19 @@ void mce_log(struct mce *mce)
         smp_wmb();
         for (;;) {
                 entry = rcu_dereference(mcelog.next);
-                /* When the buffer fills up discard new entries. Assume 
-                   that the earlier errors are the more interesting. */
-                if (entry >= MCE_LOG_LEN) {
-                        set_bit(MCE_OVERFLOW, &mcelog.flags);
-                        return;
+                for (;;) {
+                        /* When the buffer fills up discard new entries. Assume
+                           that the earlier errors are the more interesting. */
+                        if (entry >= MCE_LOG_LEN) {
+                                set_bit(MCE_OVERFLOW, &mcelog.flags);
+                                return;
+                        }
+                        /* Old left over entry. Skip. */
+                        if (mcelog.entry[entry].finished) {
+                                entry++;
+                                continue;
+                        }
                 }
-                /* Old left over entry. Skip. */
-                if (mcelog.entry[entry].finished)
-                        continue;
                 smp_rmb();
                 next = entry + 1;
                 if (cmpxchg(&mcelog.next, entry, next) == entry)
@@ -404,9 +408,15 @@ static ssize_t mce_read(struct file *filp, char __user *ubuf, size_t usize, loff
         }
 
         err = 0;
-        for (i = 0; i < next; i++) {
-                if (!mcelog.entry[i].finished)
-                        continue;
+        for (i = 0; i < next; i++) {            
+                unsigned long start = jiffies;
+                while (!mcelog.entry[i].finished) {
+                        if (!time_before(jiffies, start + 2)) {
+                                memset(mcelog.entry + i,0, sizeof(struct mce));
+                                continue;
+                        }
+                        cpu_relax();
+                }
                 smp_rmb();
                 err |= copy_to_user(buf, mcelog.entry + i, sizeof(struct mce));
                 buf += sizeof(struct mce);