linux-kernel - Re: current_thread_info() not respecting program order with gcc 4.8.x

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Message-ID: <20131119161322.GX892@tucnak.redhat.com>
Date:	Tue, 19 Nov 2013 17:13:22 +0100
From:	Jakub Jelinek <jakub@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	lttng-dev@...ts.lttng.org, Nathan Lynch <Nathan_Lynch@...tor.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: current_thread_info() not respecting program order with gcc 4.8.x

On Tue, Nov 19, 2013 at 04:57:49PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 19, 2013 at 03:29:12PM +0000, Mathieu Desnoyers wrote:
> > However, looking at ARM arch/arm/include/asm/thread_info.h:
> > 
> > static inline struct thread_info *current_thread_info(void) { register
> > unsigned long sp asm ("sp"); return (struct thread_info *)(sp &
> > ~(THREAD_SIZE - 1)); }
> > 
> > The inline assembly has no clobber and is not volatile. (this is also
> > true for all other architectures I've looked at so far, which includes
> > x86 and powerpc)

The above is not inline assembly, it is a local register variable extension,
see
http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Local-Reg-Vars.html#Local-Reg-Vars

> > Since each current_thread_info() is a different asm ("sp") without
> > clobber nor volatile, AFAIU, the compiler is within its right to
> > reorder them.

Sure.

> > One possible solution to this might be to add "memory" clobber and
> > volatile to this inline asm, but I fear it would put way too much
> > constraints on the compiler optimizations (too heavyweight).

As it is not inline asm extension, you can't.
Of course you could add asm volatile ("" : "=r" (ret) : "0" (sp));
or similar and thus make it a barrier, but I think the current
definition of current_thread_info meant to avoid all that extra
overhead.  Why does it matter when sp is different between the
current_thread_info () calls?  As long as sp & ~(THREAD_SIZE - 1) is
the same, it shouldn't make a difference.  Or is the call in between those
changing sp to something else?

	Jakub
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