Re: [Xen-devel] [PATCH LINUX v5] xen: event channel arrays are xen_ulong_t and not unsigned long

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Mar 05, 2013 at 03:56:41AM +0000, Ian Campbell wrote:
> > > diff --git a/arch/arm/include/asm/xen/events.h 
> > > b/arch/arm/include/asm/xen/events.h
> > > index 94b4e90..5c27696 100644
> > > --- a/arch/arm/include/asm/xen/events.h
> > > +++ b/arch/arm/include/asm/xen/events.h
> > > @@ -15,4 +15,26 @@ static inline int xen_irqs_disabled(struct pt_regs 
> > > *regs)
> > >   return raw_irqs_disabled_flags(regs->ARM_cpsr);
> > >  }
> > >  
> > > +/*
> > > + * We cannot use xchg because it does not support 8-byte
> > > + * values. However it is safe to use {ldr,dtd}exd directly because all
> > > + * platforms which Xen can run on support those instructions.
> > 
> > Why does atomic64_cmpxchg not work here?
> 
> Just that we don't want/need the cmp aspect, we don't mind if an extra
> bit gets set as we read the value, so long as we atomically read and set
> to zero.
> 
> > > + */
> > > +static inline xen_ulong_t xchg_xen_ulong(xen_ulong_t *ptr, xen_ulong_t 
> > > val)
> > > +{
> > > + xen_ulong_t oldval;
> > > + unsigned int tmp;
> > > +
> > > + wmb();
> > 
> > Based on atomic64_cmpxchg implementation, you could use smp_mb here
> > which avoids an outer cache flush.
> 
> Good point.
> 
> > > + asm volatile("@ xchg_xen_ulong\n"
> > > +         "1:     ldrexd  %0, %H0, [%3]\n"
> > > +         "       strexd  %1, %2, %H2, [%3]\n"
> > > +         "       teq     %1, #0\n"
> > > +         "       bne     1b"
> > > +         : "=&r" (oldval), "=&r" (tmp)
> > > +         : "r" (val), "r" (ptr)
> > > +         : "memory", "cc");
> > 
> > And a smp_mb is needed here.
> 
> I think for the specific caller which we have here it isn't strictly
> necessary, but for generic correctness I think you are right.
> 
> Thanks for reviewing.
> 
> Konrad, IIRC you have already picked this up (and sent to Linus?) so an

Yes.
> incremental fix is required? See below.

Why don't I wait a bit and wait until you are back from conferences and
can post a nice series that fixes the smp_wmb() and also the atomic one
and has been run-time tested with Xen on ARM.

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