From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> Acked-by: David S. Miller <davem@xxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
