Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751487Ab1EEFoM (ORCPT ); Thu, 5 May 2011 01:44:12 -0400 Received: from mail-wy0-f174.google.com ([74.125.82.174]:46469 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751248Ab1EEFoL (ORCPT ); Thu, 5 May 2011 01:44:11 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=CeuL7aC9BV0bs/pBW/odPID/0B+9VhWIqSXXZWSSB80TrDnvSCfvLlpmv4qwXa6Ms7 T55WOeJ1+HnASPTOlOVnzp+GexvQLBdLh1Tcp0kR4mdbA0Xw3SBVfQOzWH4L7SkjdYHn lcP+GgOpcdzcYijRBTUiD5vdEx6D06avRgU6E= Subject: Re: [PATCH] time: Add locking to xtime access in get_seconds() From: Eric Dumazet To: john stultz Cc: Andi Kleen , lkml , Paul Mackerras , "Paul E. McKenney" , Anton Blanchard , Thomas Gleixner In-Reply-To: <1304564090.2943.36.camel@work-vm> References: <1304478708-1273-1-git-send-email-john.stultz@linaro.org> <1304564090.2943.36.camel@work-vm> Content-Type: text/plain; charset="UTF-8" Date: Thu, 05 May 2011 07:44:04 +0200 Message-ID: <1304574244.32152.666.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3630 Lines: 98 Le mercredi 04 mai 2011 à 19:54 -0700, john stultz a écrit : > On Tue, 2011-05-03 at 20:52 -0700, Andi Kleen wrote: > > John Stultz writes: > > > > > From: John Stultz > > > > > > So get_seconds() has always been lock free, with the assumption > > > that accessing a long will be atomic. > > > > > > However, recently I came across an odd bug where time() access could > > > occasionally be inconsistent, but only on power7 hardware. The > > > > Shouldn't a single rmb() be enough to avoid that? > > > > If not then I suspect there's a lot more code buggy on that CPU than > > just the time. > > So interestingly, I've found that the issue was not as complex as I > first assumed. While the rmb() is probably a good idea for > get_seconds(), but it alone does not solve the issue I was seeing, > making it clear my theory wasn't correct. > > The problem was reported against the 2.6.32-stable kernel, and had not > been seen in later kernels. I had assumed the change to logarithmic time > accumulation basically reduced the window for for the issue to be seen, > but it would likely still show up eventually. > > When the rmb() alone did not solve this issue, I looked to see why the > locking did resolve it, and then it was clear: The old > update_xtime_cache() function doesn't set the xtime_cache values > atomically. > > Now, the xtime_cache writing is done under the xtime_lock, so the > get_seconds() locking resolves the issue, but isn't appropriate since > get_seconds() is called from machine check handlers. > > So the fix here for the 2.6.32-stable tree is to just update xtime_cache > in one go as done with the following patch. > > I also added the rmb() for good measure, and the rmb() should probably > also go upstream since theoretically there maybe a platform that could > do out of order syscalls. > > I suspect the reason this hasn't been triggered on x86 or power6 is due > to compiler or processor optimizations reordering the assignment to in > effect make it atomic. Or maybe the timing window to see the issue is > harder to observe? > > > Signed-off-by: John Stultz > > Index: linux-2.6.32.y/kernel/time/timekeeping.c > =================================================================== > --- linux-2.6.32.y.orig/kernel/time/timekeeping.c 2011-05-04 19:34:21.604314152 -0700 > +++ linux-2.6.32.y/kernel/time/timekeeping.c 2011-05-04 19:39:09.972203989 -0700 > @@ -168,8 +168,10 @@ int __read_mostly timekeeping_suspended; > static struct timespec xtime_cache __attribute__ ((aligned (16))); > void update_xtime_cache(u64 nsec) > { > - xtime_cache = xtime; > - timespec_add_ns(&xtime_cache, nsec); > + /* use temporary timespec so xtime_cache is updated atomically */ Atomically is not possible on 32bit platform, so this comment is misleading. What about a comment saying : /* * use temporary variable so get_seconds() cannot catch * intermediate value (one second backward) */ > + struct timespec ts = xtime; > + timespec_add_ns(&ts, nsec); > + xtime_cache = ts; > } > > /* must hold xtime_lock */ > @@ -859,6 +861,7 @@ EXPORT_SYMBOL_GPL(monotonic_to_bootbased > > unsigned long get_seconds(void) > { > + rmb(); Please dont, this makes no sense, and with no comment anyway. > return xtime_cache.tv_sec; > } > EXPORT_SYMBOL(get_seconds); > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/