linux-kernel - Re: [PATCH 2/2] xen: make xen_qlock

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Date:   Wed, 10 Oct 2018 14:47:17 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>
cc:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        boris.ostrovsky@...cle.com, hpa@...or.com, mingo@...hat.com,
        bp@...en8.de, stable@...r.kernel.org, Waiman.Long@...com,
        peterz@...radead.org
Subject: Re: [PATCH 2/2] xen: make xen_qlock_wait() nestable

On Wed, 10 Oct 2018, David Woodhouse wrote:
> On Wed, 2018-10-10 at 14:30 +0200, Thomas Gleixner wrote:
> > On Wed, 10 Oct 2018, David Woodhouse wrote:
> > 
> > > On Mon, 2018-10-01 at 09:16 +0200, Juergen Gross wrote:
> > > > -       /* If irq pending already clear it and return. */
> > > > +       /* Guard against reentry. */
> > > > +       local_irq_save(flags);
> > > > +
> > > > +       /* If irq pending already clear it. */
> > > >         if (xen_test_irq_pending(irq)) {
> > > >                 xen_clear_irq_pending(irq);
> > > > -               return;
> > > > +       } else if (READ_ONCE(*byte) == val) {
> > > > +               /* Block until irq becomes pending (or a spurious wakeup) */
> > > > +               xen_poll_irq(irq);
> > > >         }
> > > 
> > > 
> > > Does this still allow other IRQs to wake it from xen_poll_irq()?
> > > 
> > > In the case where process-context code is spinning for a lock without
> > > disabling interrupts, we *should* allow interrupts to occur still...
> > > does this?
> > 
> > Yes. Look at it like idle HLT or WFI. You have to disable interrupt before
> > checking the condition and then the hardware or in this case the hypervisor
> > has to bring you back when an interrupt is raised.
> > 
> > If that would not work then the check would be racy, because the interrupt
> > could hit and be handled after the check and before going into
> > HLT/WFI/hypercall and then the thing is out until the next interrupt comes
> > along, which might be never.
> 
> Right, but in this case we're calling into the hypervisor to poll for
> one *specific* IRQ. Everything you say is true for that specific IRQ.
> 
> My question is what happens to *other* IRQs. We want them, but are they
> masked? I'm staring at the Xen do_poll() code and haven't quite worked
> that out...

Ah, sorry. That of course has to come back like HLT/WFI for any interrupt,
but I have no idea what the Xen HV is doing there.

Thanks,

	tglx