From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755387AbZCQPUS (ORCPT ); Tue, 17 Mar 2009 11:20:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754944AbZCQPTu (ORCPT ); Tue, 17 Mar 2009 11:19:50 -0400 Received: from web.proformatique.com ([91.194.179.68]:37691 "EHLO proformatique.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755362AbZCQPTt (ORCPT ); Tue, 17 Mar 2009 11:19:49 -0400 Date: Tue, 17 Mar 2009 16:18:42 +0100 From: Guillaume Knispel To: Andrew Morton Cc: Ulrich Drepper , Ingo Molnar , Thomas Gleixner , Rusty Russell , LKML Subject: [PATCH] correct the behavior of printk_timed_ratelimit() Message-ID: <20090317161842.0059096b@xilun.lan.proformatique.com> Organization: Proformatique X-Mailer: Claws Mail 3.5.0 (GTK+ 2.12.11; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, The behavior provided by printk_timed_ratelimit() is, in some situations, probably not what a caller would reasonably expect: bool printk_timed_ratelimit(unsigned long *caller_jiffies, unsigned int interval_msecs) { if (*caller_jiffies == 0 || time_after(jiffies, *caller_jiffies)) { *caller_jiffies = jiffies + msecs_to_jiffies(interval_msecs); return true; } return false; } On a 32 bit computer, if printk_timed_ratelimit() is initially called at time jiffies == Ja, *caller_jiffies is set to Ja + msecs_to_jiffies(interval_msecs): let's say Ja + 42 for this example. If this caller then don't call printk_timed_ratelimit() until jiffies == Ja + (1 << 31) + 42 (which can happen as soon as ~ 25 days later on a 1000 HZ system), printk_timed_ratelimit() will then always return false to this caller until jiffies loops completely (1 << 31 more ticks). The proposed patch makes it only return false if jiffies is in the small time window starting at the previous call when true was returned and ending interval_msecs later. Note that if jiffies loops completely between two calls to printk_timed_ratelimit(), it will obviously still wrongly return false, but this is something with a low probability. If something completely reliable is needed I guess jiffies_64 must be used (which the proposed patch does not do). Cheers, Guillaume Knispel Avoid potential situations where printk_timed_ratelimit() can return false for no good reason during up to (1 << 31) / HZ seconds on 32 bits computers. It still can wrongly return false for a duration of up to interval_msecs if jiffies has been incremented (k * (1 << 32) + e) times -- with e < msecs_to_jiffies(interval_msecs) -- since the last call which returned true. Note that *caller_jiffies now stores jiffies just before the function returns true, instead of jiffies + msecs_to_jiffies(interval_msecs) previously. Also note, if interval_msecs is not constant for a given caller, that the interval_msecs used is now the one passed for the current call instead of previously the one used for the last call which returned true. Signed-off-by: Guillaume Knispel --- diff --git a/kernel/printk.c b/kernel/printk.c index e3602d0..2be7199 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -1292,8 +1292,11 @@ EXPORT_SYMBOL(printk_ratelimit); bool printk_timed_ratelimit(unsigned long *caller_jiffies, unsigned int interval_msecs) { - if (*caller_jiffies == 0 || time_after(jiffies, *caller_jiffies)) { - *caller_jiffies = jiffies + msecs_to_jiffies(interval_msecs); + if (*caller_jiffies == 0 + || !time_in_range(jiffies, *caller_jiffies, + *caller_jiffies + + msecs_to_jiffies(interval_msecs))) { + *caller_jiffies = jiffies; return true; } return false;