On Mon, 10 Jun 2024 14:08:16 +0530
Naveen N Rao [off-list ref] wrote:

On 32-bit powerpc, gcc generates a three instruction sequence for
function profiling:
	mflr	r0
	stw	r0, 4(r1)
	bl	_mcount

On kernel boot, the call to _mcount() is nop-ed out, to be patched back
in when ftrace is actually enabled. The 'stw' instruction therefore is
not necessary unless ftrace is enabled. Nop it out during ftrace init.

When ftrace is enabled, we want the 'stw' so that stack unwinding works
properly. Perform the same within the ftrace handler, similar to 64-bit
powerpc.

For 64-bit powerpc, early versions of gcc used to emit a three
instruction sequence for function profiling (with -mprofile-kernel) with
a 'std' instruction to mimic the 'stw' above. Address that scenario also
by nop-ing out the 'std' instruction during ftrace init.

Signed-off-by: Naveen N Rao <naveen@kernel.org>

Isn't there still the race that there's a preemption between the:

	stw	r0, 4(r1)
and
	bl	_mcount

And if this breaks stack unwinding, couldn't this cause an issue for live
kernel patching?

I know it's very unlikely, but in theory, I think the race exists.

-- Steve