linux-kernel - Re: [patch 2/3] tracing: fix sparc64 alignment crash with __u64_aligned/U64

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Message-ID: <20110122171549.GB31308@Krystal>
Date:	Sat, 22 Jan 2011 12:15:49 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	fweisbec@...il.com, mingo@...e.hu, richm@...elvet.org.uk,
	ben@...adent.org.uk, sparclinux@...r.kernel.org,
	stable <stable@...nel.org>
Subject: Re: [patch 2/3] tracing: fix sparc64 alignment crash with
	__u64_aligned/U64_ALIGN()

* David Miller (davem@...emloft.net) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Date: Fri, 21 Jan 2011 15:36:32 -0500
> 
> > Problem description:
> > 
> > gcc happily align structures defined statically on 32-byte. Ftrace trace events
> > and Tracepoints both statically define structures into sections (using the
> > "section" attribute), to then assign these to symbols with the linker scripts to
> > iterate the these sections as an array.
> > 
> > However, gcc uses different alignments for these structures when they are
> > defined statically and when they are globally visible and/or in an array.
> > Therefore iteration on these arrays sees "holes" of padding.
> > 
> > Use the __u64_aligned for type declarations and variable definitions to ensure
> > that gcc:
> > 
> > a) iterates on the correctly aligned type. (type attribute)
> > b) generates the definitions within the sections with the appropriate alignment.
> >    (variable attribute)
> > 
> > The Ftrace code introduced the "aligned(4)" variable attribute in commit
> > 1473e4417c79f12d91ef91a469699bfa911f510f to try to work around this problem that
> > showed up on x86_64, but it causes unaligned accesses on sparc64, and is
> > generally a bad idea on 64-bit if RCU pointers are contained within the
> > structure. Moreover, it did not use the same attribute as type attribute, which
> > could cause the iteration on the extern array structure not to match the
> > variable definitions for some structure sizes.
> > 
> > We should also ensure proper alignment of each Ftrace section in
> > include/asm-generic/vmlinux.lds.h.
> > 
> > Moving all STRUCT_ALIGN() for FTRACE_EVENTS() and TRACE_SYSCALLS() into the
> > definitions, so the alignment is only done if these infrastructures are
> > configured in. Use U64_ALIGN instead of STRUCT_ALIGN.
> > 
> > Also align TRACE_PRINTKS on U64_ALIGN to make sure the beginning of the section
> > is aligned on pointer size.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> 
> Acked-by: David S. Miller <davem@...emloft.net>

Thanks! This patch, just like 1/3, should also go into -stable, for both .36 and
.37.

As I'm thinking, the following patch (for Tracepoints) should only go into the
-tip tree, as it does not fix a problem -- it merely reduces the data footprint
of the tracepoints structures. I'm therefore not forwarding patch 3/3 for
inclusion into -stable.

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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