From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752745Ab0CRVrf (ORCPT ); Thu, 18 Mar 2010 17:47:35 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:53596 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752330Ab0CRVrd (ORCPT ); Thu, 18 Mar 2010 17:47:33 -0400 From: John Stultz To: lkml Cc: John Stultz , Thomas Gleixner , Sonic Zhang Subject: [PATCH] time: Fix accumulation bug triggered by long delay. Date: Thu, 18 Mar 2010 14:47:30 -0700 Message-Id: <1268948850-5225-1-git-send-email-johnstul@us.ibm.com> X-Mailer: git-send-email 1.6.0.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The logarithmic accumulation done in the timekeeping has some overflow protection that limits the max shift value. That means it will take more then shift loops to accumulate all of the cycles. This causes the shift decrement to underflow, which causes the loop to never exit. The simplest fix would be simply to do a: if (shift) shift--; However that is not optimal, as we know the cycle offset is larger then the interval << shift, the above would make shift drop to zero, then we would be spinning for quite awhile accumulating at interval chunks at a time. Instead, this patch only decreases shift if the offset is smaller then cycle_interval << shift. This makes sure we accumulate using the largest chunks possible without overflowing tick_length, and limits the number of iterations through the loop. This issue was found and reported by Sonic Zhang, who also tested the fix. Many thanks your explanation and testing! CC: Thomas Gleixner CC: Sonic Zhang Signed-off-by: John Stultz --- kernel/time/timekeeping.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 1673637..39f6177 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -818,7 +818,8 @@ void update_wall_time(void) shift = min(shift, maxshift); while (offset >= timekeeper.cycle_interval) { offset = logarithmic_accumulation(offset, shift); - shift--; + if(offset < timekeeper.cycle_interval<