linux-kernel - Re: [RFC PATCH] arm64: cmpxchg.h: Bring ldxr and stxr closer

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Date:	Tue, 3 Mar 2015 14:34:54 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	Steve Capper <steve.capper@...aro.org>,
	open list <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] arm64: cmpxchg.h: Bring ldxr and stxr closer

On Fri, Feb 27, 2015 at 03:29:38PM -0500, Pranith Kumar wrote:
> On Fri, Feb 27, 2015 at 3:17 PM, Pranith Kumar <bobby.prani@...il.com> wrote:
> > On Fri, Feb 27, 2015 at 3:15 PM, Will Deacon <will.deacon@....com> wrote:
> >>> @@ -166,11 +166,11 @@ static inline int __cmpxchg_double(volatile void *ptr1, volatile void *ptr2,
> >>>               VM_BUG_ON((unsigned long *)ptr2 - (unsigned long *)ptr1 != 1);
> >>>               do {
> >>>                       asm volatile("// __cmpxchg_double8\n"
> >>> +                     "       mov     %w0, #0\n"
> >>>                       "       ldxp    %0, %1, %2\n"
> >>
> >> Seriously, you might want to test this before you mindlessly make changes to
> >> low-level synchronisation code. Not only is the change completely unnecessary
> >> but it is actively harmful.
> >>
> >
> > Oops, I apologize for this. I should have looked more closely. It is
> > wrong to do this in cmpxchg_double(). What about the other cases?
> 
> I tried looking closely on what might be the problem here. I am
> waiting on a HiKey arm64 board and I agree I should not send in
> changes without running/testing them first.
> 
> Could you please explain (for educational purposes) why you think this
> change is harmful?

Do you mean the cmpxchg_double() change? Becuase %w0 and %0 is the same
physical register. You set it to 0 and immediately override it with
ldxp.

-- 
Catalin
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