linux-kernel - Re: [PATCH 1/5] xen/spinlock: Fix locking path engaging too soon under PVHVM.

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Date:	Mon, 9 Sep 2013 11:31:23 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
CC:	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <stefan.bader@...onical.com>,
	<stefano.stabellini@...citrix.com>, <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 1/5] xen/spinlock: Fix locking path engaging too soon
 under PVHVM.

On 07/09/13 14:46, Konrad Rzeszutek Wilk wrote:
> The xen_lock_spinning has a check for the kicker interrupts
> and if it is not initialised it will spin normally (not enter
> the slowpath).
> 
> But for PVHVM case we would initialise the kicker interrupt
> before the CPU came online. This meant that if the booting
> CPU used a spinlock and went in the slowpath - it would
> enter the slowpath and block forever. The forever part b/c

b/c? Ewww.  Proper English please.

> during bootup the interrupts are disabled - so the CPU would
> never get an IPI kick and would stay stuck in the slowpath
> logic forever.

This description isn't right -- VCPUs blocked in SCHEDOP_poll can be
unblocked on the event they're waiting for even if local irq delivery is
disabled.

> Why would the booting CPU never get an IPI kick? B/c in both
> PV and PVHVM we consult the cpu_online_mask to determine whether
> the IPI should go to its CPU destination. Since the booting
> CPU has not yet finished and set that flag, it meant that
> if any spinlocks were taken before the booting CPU had gotten to:

I can't find where the online mask is being checked in
xen_send_IPI_one().  Is this really the reason why it didn't work?

This fix looks fine but both the description and the comment need to be
fixed/clarified.

David
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