From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH] x86/HPET: mask interrupt while changing
	affinity
Date: Wed, 20 Mar 2013 13:46:06 +0000
Message-ID: <5149BD9E.60502@citrix.com>
References: <514704C202000078000C64A0@nat28.tlf.novell.com>
	<1701498510.20130319165336@eikelenboom.it>
	<514899B102000078000C6D74@nat28.tlf.novell.com>
	<135661008.20130319234805@eikelenboom.it>
	<51497FD902000078000C708A@nat28.tlf.novell.com>
	<704281841.20130320111312@eikelenboom.it>
	<5149A53802000078000C71DB@nat28.tlf.novell.com>
	<12710184253.20130320125556@eikelenboom.it>
	<5149CA0202000078000C72BD@nat28.tlf.novell.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
In-Reply-To: <5149CA0202000078000C72BD@nat28.tlf.novell.com>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: Jan Beulich <JBeulich@suse.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>, "Keir (Xen.org)" <keir@xen.org>, xen-devel <xen-devel@lists.xen.org>
List-Id: xen-devel@lists.xenproject.org

On 20/03/13 13:38, Jan Beulich wrote:
>>>> On 20.03.13 at 12:55, Sander Eikelenboom <linux@eikelenboom.it> wrote:
>> Close but not entirely ;)
> Close to not crashing, maybe, but whether this really helps with your
> problem is still entirely unclear.
>
>> See attached serial log
> Okay, I wasn't even aware of that assertion in _spin_lock_irq().
>
> Keir, do you really think this is necessary? In the prior patch
> version, handle_hpet_broadcast() had a flow like this

As I have been playing with spinlocks and the recursive NMI path, I
would say that the assertion is necessary, given the other callsites of
spin_{un,}lock_irq()

>
>     spin_lock_irqsave();
>     ...
>     spin_unlock_irqrestore();
>     ...
>     if ( next_event != STIME_MAX )
>     {
>         spin_lock_irq();
>         ...
>         spin_unlock_irqrestore();
>     }
>
> avoiding the saving of the flags in the second lock acquire. Said
> assertion makes it impossible to do this.

I would agree that in this case the logic is correct despite the assertion.

Overall, I would argue that obviously correct matching locking pairs is
more important the performance penalty from an additional pushf & mov

~Andrew

>
> Sander, in any case, attached a fixed version of the patch (I had
> to guess which of the two spin_lock_irq() calls it was, as the log
> was incomplete in that the stack trace got dropped, but am pretty
> positive that it was the one in handle_hpet_broadcast()).
>
> Jan
>