Re: [Xen-devel] [PATCH] xen: arm: clear the exclusive monitor on exception return

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-07-18 at 17:48 +0100, Tim Deegan wrote:
> At 09:31 +0100 on 18 Jul (1374139884), Ian Campbell wrote:
> > On Wed, 2013-07-17 at 20:48 +0100, Tim Deegan wrote:
> > > At 12:18 +0100 on 17 Jul (1374063531), Ian Campbell wrote:
> > > > Otherwise context switching between two vcpus which are contending the 
> > > > same
> > > > lock can result in a spurious success.
> > > 
> > > Shouldn't this go in ctxt_swicth_to(), then?  I think any use that Xen
> > > itself makes of the monitor should end with it cleared.
> > 
> > Our atomic read/set which we stole^Winherited from Linux relies on this
> > behaviour (interlocking with atomic_add_and_blah stuff).
> >         /*
> >          * On ARM, ordinary assignment (str instruction) doesn't clear the 
> > local
> >          * strex/ldrex monitor on some implementations. The reason we can 
> > use it for
> >          * atomic_set() is the clrex or dummy strex done on every exception 
> > return.
> >          */
> 
> Ah, OK.  So, Ack, but if it's not already committed can you edit the
> commit message to refer to this?

Applied, thanks. Commit message is now:

        xen: arm: clear the exclusive monitor on exception return
        
        Otherwise context switching between two vcpus which are contending the 
same
        lock can result in a spurious success.
        
        Our spinlock and atomics code (which we get from Linux) rely on this 
behaviour
        because they use non-exclusive stores for single instruction operations 
(e.g. 
        spin_unlock or atomic_set).
        
        This is not required on ARMv8 since eret implicitly clears the monitor.
        
        Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
        Acked-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
        Acked-by: Tim Deegan <tim@xxxxxxx>
        


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