> > 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
incremental fix is required? See below.
Ian.
8<------------------------------------
>From 4ed928274dad4c3ed610e769b2ae11eb2d1ea433 Mon Sep 17 00:00:00 2001
From: Ian Campbell <i...@hellion.org.uk>
Date: Tue, 5 Mar 2013 03:37:23 +0000
Subject: [PATCH] arm: xen: correct barriers in xchg_xen_ulong
We can use an smp_wmb rather than a wmb here and we also need one after the
exchange. Spotted by Rob Herring.
Signed-off-by: Ian Campbell <i...@hellion.org.uk>
---
arch/arm/include/asm/xen/events.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/arm/include/asm/xen/events.h
b/arch/arm/include/asm/xen/events.h
index 5c27696..0e1f59e 100644
--- a/arch/arm/include/asm/xen/events.h
+++ b/arch/arm/include/asm/xen/events.h
@@ -25,7 +25,7 @@ static inline xen_ulong_t xchg_xen_ulong(xen_ulong_t *ptr,
xen_ulong_t val)
xen_ulong_t oldval;
unsigned int tmp;
- wmb();
+ smp_wmb();
asm volatile("@ xchg_xen_ulong\n"
"1: ldrexd %0, %H0, [%3]\n"
" strexd %1, %2, %H2, [%3]\n"
@@ -34,6 +34,7 @@ static inline xen_ulong_t xchg_xen_ulong(xen_ulong_t *ptr,
xen_ulong_t val)
: "=&r" (oldval), "=&r" (tmp)
: "r" (val), "r" (ptr)
: "memory", "cc");
+ smp_wmb();
return oldval;
}
--
1.7.10.4
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