From 4f84e4be53a04a65d97bf0faa0c8f99e29bc0170 Mon Sep 17 00:00:00 2001 From: Joshua Wise Date: Sat, 23 Jun 2007 17:16:45 -0700 Subject: [PATCH] x86_64: fix misplaced `continue' in mce.c Background: When a userspace application wants to know about machine check events, it opens /dev/mcelog and does a read(). Usually, we found that this interface works well, but in some cases, when the system was taking large numbers of machine check exceptions, the read() would hang. The system would output a soft-lockup warning, and the daemon reading from /dev/mcelog would suck up as much of a single CPU as it could spinning in system space. Description: This patch fixes this bug. In particular, there was a "continue" inside a timeout loop that presumably was intended to break out of the outer loop, but instead caused the inner loop to continue. This patch also makes the condition for the break-out a little more evident by changing a !time_before to a time_after_eq. Result: The read() no longer hangs in this test case. Testing: On my system, I could replicate the bug with the following command: # for i in `seq 15000`; do ./inject_sbe.sh; done where inject_sbe.sh contains commands to inject a single-bit error into the next memory write transaction. Patch: This patch is against git f1518a088bde6aea49e7c472ed6ab96178fcba3e. Signed-off-by: Joshua Wise Signed-off-by: Tim Hockin Cc: Andi Kleen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/x86_64/kernel/mce.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86_64/kernel/mce.c b/arch/x86_64/kernel/mce.c index a14375dd54..aa1d159917 100644 --- a/arch/x86_64/kernel/mce.c +++ b/arch/x86_64/kernel/mce.c @@ -497,15 +497,17 @@ static ssize_t mce_read(struct file *filp, char __user *ubuf, size_t usize, loff for (i = 0; i < next; i++) { unsigned long start = jiffies; while (!mcelog.entry[i].finished) { - if (!time_before(jiffies, start + 2)) { + if (time_after_eq(jiffies, start + 2)) { memset(mcelog.entry + i,0, sizeof(struct mce)); - continue; + goto timeout; } cpu_relax(); } smp_rmb(); err |= copy_to_user(buf, mcelog.entry + i, sizeof(struct mce)); buf += sizeof(struct mce); + timeout: + ; } memset(mcelog.entry, 0, next * sizeof(struct mce)); -- 2.39.5