linux-kernel - Re: [PATCH v5 3/4] kvm: Create kvm_clear

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Date:	Wed, 18 Jul 2012 13:33:35 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, avi@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v5 3/4] kvm: Create kvm_clear_irq()

On Wed, Jul 18, 2012 at 01:27:39PM +0300, Gleb Natapov wrote:
> On Wed, Jul 18, 2012 at 01:20:29PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jul 18, 2012 at 09:27:42AM +0300, Gleb Natapov wrote:
> > > On Tue, Jul 17, 2012 at 07:14:52PM +0300, Michael S. Tsirkin wrote:
> > > > > _Seems_ racy, or _is_ racy?  Please identify the race.
> > > > 
> > > > Look at this:
> > > > 
> > > > static inline int kvm_irq_line_state(unsigned long *irq_state,
> > > >                                      int irq_source_id, int level)
> > > > {
> > > >         /* Logical OR for level trig interrupt */
> > > >         if (level)
> > > >                 set_bit(irq_source_id, irq_state);
> > > >         else
> > > >                 clear_bit(irq_source_id, irq_state);
> > > > 
> > > >         return !!(*irq_state);
> > > > }
> > > > 
> > > > 
> > > > Now:
> > > > If other CPU changes some other bit after the atomic change,
> > > > it looks like !!(*irq_state) might return a stale value.
> > > > 
> > > > CPU 0 clears bit 0. CPU 1 sets bit 1. CPU 1 sets level to 1.
> > > > If CPU 0 sees a stale value now it will return 0 here
> > > > and interrupt will get cleared.
> > > > 
> > > This will hardly happen on x86 especially since bit is set with
> > > serialized instruction.
> > 
> > Probably. But it does make me a bit uneasy.  Why don't we pass
> > irq_source_id to kvm_pic_set_irq/kvm_ioapic_set_irq, and move
> > kvm_irq_line_state to under pic_lock/ioapic_lock?  We can then use
> > __set_bit/__clear_bit in kvm_irq_line_state, making the ordering simpler
> > and saving an atomic op in the process.
> > 
> With my patch I do not see why we can't change them to unlocked variant
> without moving them anywhere. The only requirement is to not use RMW
> sequence to set/clear bits. The ordering of setting does not matter. The
> ordering of reading is.

You want to use __set_bit/__clear_bit on the same word
from multiple CPUs, without locking?
Why won't this lose information?

In any case, it seems simpler and safer to do accesses under lock
than rely on specific use.

> --
> 			Gleb.
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