On Tue, Sep 22, 2015 at 03:57:06PM +0200, Hans-Peter Nilsson wrote: > > From: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> > > Date: Tue, 22 Sep 2015 15:27:51 +0200 > > > On Tue, Sep 22, 2015 at 02:50:19PM +0200, Hans-Peter Nilsson wrote: > > > That element (the struct) needs *explicit* padding or alignment > > > to the required multiplicity of bytes for anyone to portably be > > > able to imply something other than "byte alignment" for the > > > layout of it, as elements of an array, across systems. Use > > > dummy elements or a compiler construct like __attribute__ > > > ((__aligned__ (...))) per kernel policy or taste. I'd recommend > > > specifying the alignment, so TRT will happen for it when it in > > > turn is an element of an otherwise unpadded struct. > > > I see. I would say it's very risky ABI choice, but okay. > > > > What was the reason behind? I don't understand it. > > It was made some 20+ years ago, some of the reason being (here's > the irony) compatibility with a toolchain for another > architecture, popular at the time, now forgotten. > Another reason (IIRC) was that it saves space. :) > > > Is it free to make misaligned memory access on CRIS? > > Within a cache-line for CRIS v32, it's free. > > > What about atomicity? How it works for misaligned accesses? > > Good spotting. No system with page layouts fixed at size > multiples (all are, it'd be crazy to split pages as low as byte > boundaries) can support naturally-misaligned atomic accesses. > > Therefore elements with access expecting atomicity, have be > decorated with alignment-inducing attributes, for portability, > e.g. to work for CRIS. In userspace, I can at times get away > with calling a special function with a process-wide lock, in > those cases where the upstream project is unlikely to timely > understand e.g. that a naked "int" is not naturally aligned. > > > The patch below fixes issue for me. > > Thanks. > > > I'm not sure if we want to ask for alignment to sizeof(long). > > aligned(2) works too. > > I guess you hit the right spot, but I'd think people would be > more comfortable with aligning to sizeof (void *). I would indeed prefer sizeof(void *). Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
