On Tue, Oct 27, 2015 at 03:44:24PM +0000, Will Deacon wrote:
> On Fri, Oct 16, 2015 at 11:37:29AM +0100, Catalin Marinas wrote:
> > On Fri, Oct 16, 2015 at 08:52:24AM +0100, Vladimir Murzin wrote:
> > > > On 15/10/15 14:02, Will Deacon wrote:
> > > >> diff --git a/arch/arm64/kernel/armv8_deprecated.c
> > > >> b/arch/arm64/kernel/armv8_deprecated.c
> > > >> index bcee7abac68e..6039d1eb5912 100644
> > > >> --- a/arch/arm64/kernel/armv8_deprecated.c
> > > >> +++ b/arch/arm64/kernel/armv8_deprecated.c
> > > >> @@ -284,12 +284,12 @@ static void
> > > >> register_insn_emulation_sysctl(struct ctl_table *table)
> > > >> __asm__ __volatile__( \
> > > >> ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN, \
> > > >> CONFIG_ARM64_PAN) \
> > > >> - " mov %w2, %w1\n" \
> > > >> - "0: ldxr"B" %w1, [%3]\n" \
> > > >> - "1: stxr"B" %w0, %w2, [%3]\n" \
> > > >> + "0: ldxr"B" %w2, [%3]\n" \
> > > >> + "1: stxr"B" %w0, %w1, [%3]\n" \
> > > >> " cbz %w0, 2f\n" \
> > > >> " mov %w0, %w4\n" \
> > > >> "2:\n" \
> > > >> + " mov %w1, %w2\n" \
> > > >> " .pushsection .fixup,\"ax\"\n" \
> > > >> " .align 2\n" \
> > > >> "3: mov %w0, %w5\n" \
> > >
> > > On the second thought looks like we still update *data in case stxr
> > > fails (or I need more coffee).
> >
> > I'm on the second cup and I see the same problem. Even if stxr fails, we
> > fall back through "mov %w1, %w2", so *data is always updated with the
> > loaded value. Maybe something like below on top of Will's patch:
>
> Yeah, sorry, my original patch was an untested mess. I think we can avoid
> the "cc" clobber by adding a branch to the slow-path, as below.
>
> Still needs testing, mind.
Right, it passes the simple test case I wrote (which fails under mainline).
Will
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