* David Miller ([email protected]) wrote: > From: Mathieu Desnoyers <[email protected]> > 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 <[email protected]> > > Acked-by: David S. Miller <[email protected]>
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 [email protected] (the same should be done for the two other patches in this series). Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com _______________________________________________ stable mailing list [email protected] http://linux.kernel.org/mailman/listinfo/stable
