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

[David Vrabel <david.vrabel@citrix.com>]

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