From mboxrd@z Thu Jan 1 00:00:00 1970 From: Petr Tesarik Date: Fri, 27 Aug 2010 14:40:48 +0000 Subject: Re: Serious problem with ticket spinlocks on ia64 Message-Id: <201008271640.49542.ptesarik@suse.cz> List-Id: References: <201008271537.35709.ptesarik@suse.cz> <201008271609.16099.ptesarik@suse.cz> <20100827143135.GY18967@zorg.emea.sgi.com> In-Reply-To: <20100827143135.GY18967@zorg.emea.sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Hedi Berriche Cc: linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, Tony Luck On Friday 27 of August 2010 16:31:35 Hedi Berriche wrote: > On Fri, Aug 27, 2010 at 15:09 Petr Tesarik wrote: > | On Friday 27 of August 2010 15:48:02 Hedi Berriche wrote: > | > One more fact, the problem was introduced by commit > | > > | > commit 9d40ee200a527ce08ab8c793ba8ae3e242edbb0e > | > Author: Tony Luck > | > Date: Wed Oct 7 10:54:19 2009 -0700 > | > > | > [IA64] Squeeze ticket locks back into 4 bytes. > | > > | > Reverting the patch makes the problem go away. > | > > | > IOW, and as far as testing shows, the first incarnation of the ticket > | > locks implementation on IA64 (commit 2c8696), the one that used 8 > | > bytes, does not exhibit this problem. > | > | I wouldn't be so sure about it. Given that I have only observed the > | problem when the spinlock value wraps around, then an 8-byte spinlock > | might only need much more time to trigger the bug. > > That's a possibility and that's why I said "as far as testing shows". > > That said, I'm letting my already over 36 hours run carry on chewing CPU > time, and see if it will eventually trip the same problem seen with 4-byte > ticket locks. Hm, this doesn't sound like a viable approach. Since the siglock gets initialized to 0 when a new process is started, it may never actually wrap around. I would rather attach a SystemTap probe somewhere during process fork and add a bias to the siglock. That should work fine. Let me knock up the SystemTap script... Petr