linux-kernel - Re: sequence lock in Linux

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Message-ID: <4C12A539.1000709@zytor.com>
Date:	Fri, 11 Jun 2010 14:06:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org
Subject: Re: sequence lock in Linux

On 06/11/2010 01:36 PM, Paul E. McKenney wrote:
> 
> The reason that the C standard permits this is to allow for things like
> 8-bit CPUs, which are simply unable to load or store 32-bit quantities
> except by doing it chunkwise.  But I don't expect the Linux kernel to
> boot on these, and certainly not on any of the ones that I have used!
> 
> I most definitely remember seeing a gcc guarantee that loads and stores
> would be done in one instruction whenever the hardware supported this,
> but I am not finding it today.  :-(
> 

What gcc does not -- and should not -- guarantee is that accessing a
non-volatile member is done exactly once.  As Mathieu pointed out, it
can choose to drop it due to register pressure and load it again.

What is possibly a much bigger risk -- since this is an inline -- is
that the value is cached from a previous piece of code, *or* that since
the structure is const(!) that the second read in the repeat loop is
elided.  Presumably current versions of gcc don't do that across a
memory clobber, but that doesn't seem entirely out of the question.

	-hpa
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