linux-kernel - RE: Serious problem with ticket spinlocks on ia64

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Message-ID: <987664A83D2D224EAE907B061CE93D53015D91D6F5@orsmsx505.amr.corp.intel.com>
Date:	Fri, 27 Aug 2010 22:01:18 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Hedi Berriche <hedi@....com>
CC:	Petr Tesarik <ptesarik@...e.cz>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Serious problem with ticket spinlocks on ia64

>*Hasty* look seems to suggest that keeping the fancy asm bits but compiling >with
>-O2 -frename-registers makes it work equally well.
>
>Don't take my word for it though, double check it, it's been a long week and
>brain is anything but alert at this time of the night.

Yup - that makes the usermode version run just fine (no problems
in the first billion lock/unlock operations ... which is
quite a lot of wrap-arounds of the 15-bit ticket lock).

And since we already use -frename-registers when building the
kernel, no immediate help for the kernel problem. :-(

I may tinker with this test a bit to include some short random
amounts of hold-time for the lock, and delays between attempts
to acquire it (to make it look more like a contended kernel lock
and less like a continuous queue of processes trading around a
lock that is never free ... Petr's debug information definitely
showed the lock becoming free at the wraparound (lock == 0x0).

-Tony
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