Re: [patch 1/3] introduce __u64_aligned and U64_ALIGN() for structure alignment in custom sections (v3)

Mathieu Desnoyers
2011-01-22T17:11:39Z

* David Miller (davem@davemloft.net) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Date: Fri, 21 Jan 2011 15:36:31 -0500
>
> > Problem description:
> >
> > gcc happily align on 32-byte structures defined statically. Ftrace trace events
> > and Tracepoints both statically define structures into custom 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 than when they are globally visible and/or in an array.
> > Therefore iteration on these arrays sees "holes" of padding. gcc is within its
> > rights to increase the alignment of the statically defined structures because,
> > normally, there should be no other accesses to them than in the local object. We
> > are actually iterating on the generated structures as if they were an array
> > without letting gcc knowing anything about it.
> >
> > This patch introduces __u64_aligned to force gcc to use the u64 type and
> > variable alignment, up-aligning or down-aligning the target type if necessary.
> > The memory accesses to the target structure are efficient (does not require
> > bytewise memory accesses) and the atomic pointer update guarantees required by
> > RCU are kept. u64 is considered as the largest type that can generate a trap for
> > unaligned accesses (u64 on sparc32 needs to be aligned on 64-bit).
> >
> > This alignment should be used for both structure definitions and declarations
> > (as *both* the type and variable attribute) when using the "section"
> > attribute to generate arrays of structures. Given that gcc only uses the type
> > attribute "aligned" as a lower-bound for alignment, the structures should not
> > contain types which require alignment larger than that of u64. The "aligned"
> > variable attribute, on the other hand, forces gcc to use exactly the specified
> > alignment.
> >
> > Also introduce the linker script U64_ALIGN() macro for specification of custom
> > section alignment that matches that of __u64_aligned.
> >
> > Changelog since v2:
> > - Drop the "packed" type attribute, because it causes gcc to drop the padding
> > between consecutive "int" and "pointer"/"long" fields, which leads to
> > unaligned accesses on sparc64.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>
> Acked-by: David S. Miller <davem@davemloft.net>

Thanks David,

After these patches get merged -- I would recommend going through the tracing
tree -- they'll have to go to -stable too. I'm therefore forwarding this to
stable@kernel.org (the same should be done for the two other patches in this
series).

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.