On Tue, 11 Jun 2024 20:17:19 +0530
Naveen N Rao <nav...@kernel.org> wrote:
> > I know it's very unlikely, but in theory, I think the race exists.
>
> I *think* you are assuming that we will be patching back the 'stw'
Yes, that was what I was assuming :-p
> instruction here? So, there could be an issue if a cpu has executed the
> nop instead of 'stw' and then sees the call to _mcount().
>
> But, we don't patch back the 'stw' instruction. That is instead done as
> part of ftrace_caller(), along with setting up an additional stack frame
> to ensure reliable stack unwinding. Commit 41a506ef71eb
> ("powerpc/ftrace: Create a dummy stackframe to fix stack unwind") has
> more details.
>
> The primary motivation for this patch is to address differences in the
> function profile sequence with various toolchains. Since commit
> 0f71dcfb4aef ("powerpc/ftrace: Add support for
> -fpatchable-function-entry"), we use the same two-instruction profile
> sequence across 32-bit and 64-bit powerpc:
> mflr r0
> bl ftrace_caller
>
> This has also been true on 64-bit powerpc with -mprofile-kernel, except
> the very early versions of gcc that supported that option (gcc v5).
>
> On 32-bit powerpc, we used to use the three instruction sequence before
> support for -fpatchable-function-entry was introduced.
>
> In this patch, we move all toolchain variants to use the two-instruction
> sequence for consistency.
OK, if you are not patching that back, then all should be good.
-- Steve
