On Mon, 13 Jan 2025, Magnus Lindholm wrote:
> I've been running some more tests, this time with a smp kernel but on
> a system with just one cpu, seems to me as a bit simpler scenario to
> analyze. I've added some print statements to smp_call_function_single,
> just to see what's really going on:
>
> pr_warn("smp_call_function_single: %llx %llx
> size=%d\n",&csd_stack,&csd, sizeof(call_single_data_t));
>
> output is seen below:
> smp: smp_call_function_single: fffffc000493fc40 fffffc000493fc58 size=32
> so, the csd_stack struct is 32-bytes in size but &csd - &csd_stack =
> 24. This does not make any sense?
Given information supplied previously it does, see below.
> When requesting csd_stack to be aligned using
> __attribute__((__aligned__(x))) it seems as if the compiler does not
> leave enough room above the csd_stack struct. i.e since the exact
> location of csd_stack depends on the actual value of $sp it is not
> known at compile time. Seems like gcc does not take this into account.
> The code works fine if I remove the alignment attribute for csd_stack.
> Also as previously mentioned, declaring csd_stack as "struct
> ____cacheline_aligned_in_smp" makes it work, but judging from the
> disassembly code, this statement has no effect on the alignment of
> csd_stack, i.e csd_stack is not aligned to anything its simply just
> placed on the stack, indirectly making it just 16-byte aligned instead
> of the requested 32-byte alignment.
>
> It seems to me that, when used to align variables that reside on the
> stack, __attribute__((__aligned__(x))) does not work correctly with
> gcc/alpha/linux.
I smell psABI breakage somewhere causing stack misalignment upframe. It
has happened before here and there. It could genuinely be a GCC bug, but
I suspect not. I'd rather suspect handcoded assembly or other kind of a
manual stack pointer assignment or adjustment made somewhere.
Can you please retrieve the value of SP in `smp_call_function_single'?
Just something such as:
printk("SP: %016lx\n", __builtin_frame_address(0));
should do. If it does show SP as unaligned, then we can dig in deeper.
Maciej
