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

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Message-ID: <20120718105105.GG4700@redhat.com>
Date:	Wed, 18 Jul 2012 13:51:05 +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 01:36:08PM +0300, Gleb Natapov wrote:
> On Wed, Jul 18, 2012 at 01:33:35PM +0300, Michael S. Tsirkin wrote:
> > 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?
> Because it is not RMW. If it is then yes, you can't do that.

You are saying __set_bit does not do RMW on x86? Interesting.
It's probably not a good idea to rely on this I think.

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