linux-kernel - Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail

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Message-ID: <20150227193358.GF9011@leverpostej>
Date:	Fri, 27 Feb 2015 19:33:58 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	Steve Capper <steve.capper@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	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: Clear the exclusive access bit on
 fail

On Fri, Feb 27, 2015 at 07:15:57PM +0000, Pranith Kumar wrote:
> On Fri, Feb 27, 2015 at 2:08 PM, Mark Rutland <mark.rutland@....com> wrote:
> > On Fri, Feb 27, 2015 at 06:44:19PM +0000, Pranith Kumar wrote:
> >> On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas
> >> <catalin.marinas@....com> wrote:
> >> > It's either badly formatted or I don't get it. Are the "stxr x1" and
> >> > "stxr x7" happening on the same CPU (P0)? If yes, that's badly written
> >> > code, not even architecturally compliant (you are not allowed other
> >> > memory accesses between ldxr and stxr).
> >>
> >> OK. Is that the same case with ldaxr (acquire) and stlxr (release)?
> >> AFAIK, memory accesses between acquire and release exclusive
> >> operations are allowed.
> >
> > The restriction on memory accesses in the middle of a load-exclusive
> > store-exclusive sequence applies to all the load/store-exclusive
> > variants, including ldaxr and stlxr.
> >
> 
> Thanks Mark. I am trying to see where this restriction is documented.
> Looking at: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/a64_data_transfer_alpha.html
> I do not see that mentioned. The only restriction relevant is that a
> stxr should use the same address as the most recent ldxr.
> 
> Could you please point me to the relevant documentation?

You will need to look at the latest ARMv8-A Architecture Reference
Manual [1].

Table B2-2 defines the set of Load-Exclusive and Store-Exclusive
variants, including LDAXR and STLXR.

Taking this into account, take a look at the restrictions in section
B2.10.5 "Load-Exclusive and Store-Exclusive instruction usage
restrictions". One of the bullet points nodes that software must avoid
explicit memory accesses between a Load-Exclusive instruction and the
associated Store-Exclusive.

Thanks,
Mark.

[1] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0487a.e/index.html
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