From mboxrd@z Thu Jan 1 00:00:00 1970 From: Srivatsa Vaddagiri Subject: Re: [PATCH RFC 09/12] xen/pvticketlock: Xen implementation for PV ticket locks Date: Tue, 18 Jan 2011 21:57:18 +0530 Message-ID: <20110118162717.GA18234@linux.vnet.ibm.com> References: <32e63cc978ec4b3f36c7f641ce48b3d86aed22ed.1279328276.git.jeremy.fitzhardinge@citrix.com> <20100926113910.GA6719@linux.vnet.ibm.com> <4C9FCA8F.4070802@goop.org> Reply-To: vatsa@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4C9FCA8F.4070802@goop.org> Sender: linux-kernel-owner@vger.kernel.org To: Jeremy Fitzhardinge Cc: Linux Kernel Mailing List , Nick Piggin , Peter Zijlstra , Jan Beulich , Avi Kivity , Xen-devel , suzuki@in.ibm.com, Konrad Rzeszutek Wilk List-Id: xen-devel@lists.xenproject.org On Sun, Sep 26, 2010 at 03:34:55PM -0700, Jeremy Fitzhardinge wrote: > On 09/26/2010 04:39 AM, Srivatsa Vaddagiri wrote: > > On Fri, Jul 16, 2010 at 06:03:04PM -0700, Jeremy Fitzhardinge wrote: > >> Replace the old Xen implementation of PV spinlocks with and implementation > >> of xen_lock_spinning and xen_unlock_kick. > > I see that the old implementation took care of a spinlock() call being > > interrupted by another spinlock (in interrupt handler), by saving/restoring > > old lock of interest. We don't seem to be doing that in this new version? > > Won't that lead to loss of wakeup -> hang? Sorry about coming back late on this, but as I was looking at the most recent version of pv-ticketlocks, this came up in my mind again .. > No, interrupts are disabled while waiting to take the lock, so it isn't > possible for an interrupt to come in. Where are we disabling interrupts? Is it in xen_poll_irq()? > With the old-style locks it was > reasonable to leave interrupts enabled while spinning, but with ticket > locks it isn't. > > (I haven some prototype patches to implement nested spinning of ticket > locks, Hmm ..where is nested spinning allowed/possible? Process context will disable interrupts/bh from wanting the same (spin-)lock it is trying to acquire? > by allowing the nested taker to steal the queue position of the > outer lock-taker, and switch its ticket with a later one. But there's a > fundamental problem with the idea: each lock taker needs to take a > ticket. If you don't allow nesting, then the max amount of tickets > needed = number of cpus-1; however, with nesting, the max number of > tickets = ncpus * max-nesting-depth, so the size of the ticket type must > be larger for a given number of cpus, or the max number of cpus must be > reduced.) - vatsa