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From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     linux-kernel@vger.kernel.org
Cc:     Greentime Hu <green.hu@gmail.com>, Nick Hu <nickhu@andestech.com>,
        Vincent Chen <deanbo422@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: nds32: Fails to compile/ not expected $r25 usage.
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Hi,

I run into:
|  CC      arch/nds32/kernel/perf_event_cpu.o
|In file included from include/linux/sched.h:12,
|                 from include/linux/ptrace.h:6,
|                 from include/uapi/asm-generic/bpf_perf_event.h:4,
|                 from ./arch/nds32/include/generated/uapi/asm/bpf_perf_eve=
nt.h:1,
|                 from include/uapi/linux/bpf_perf_event.h:11,
|                 from include/linux/perf_event.h:18,
|                 from arch/nds32/kernel/perf_event_cpu.c:9:
|arch/nds32/kernel/perf_event_cpu.c: In function 'unwind_frame_kernel':
|arch/nds32/include/asm/current.h:10:13: error: '$r25' undeclared (first us=
e in this function)
|   10 | #define tsk $r25
|      |             ^~~~
|arch/nds32/kernel/perf_event_cpu.c:1255:50: note: in expansion of macro 't=
sk'
| 1255 |                                                 (tsk, &graph, fram=
e->lp, NULL);
|      |                                                  ^~~
|arch/nds32/include/asm/current.h:10:13: note: each undeclared identifier i=
s reported only once for each function it appears in
|   10 | #define tsk $r25
|      |             ^~~~
|arch/nds32/kernel/perf_event_cpu.c:1255:50: note: in expansion of macro 't=
sk'
| 1255 |                                                 (tsk, &graph, fram=
e->lp, NULL);
|      |                                                  ^~~
|arch/nds32/kernel/perf_event_cpu.c:1257:27: warning: assignment to 'long u=
nsigned int' from 'long unsigned int *' makes integer from pointer without =
a cast [-Wint-conversion]
| 1257 |                 frame->sp =3D ((unsigned long *)frame->sp) + 1;
|      |                           ^

Using `current' instead of `tsk' works and might be the right thing to
do here.
I recommend to avoid using
| #define tsk $r25

since it will clash with its usage as a variable. I am actually
surprised that neither kernel/exit.c nor kernel/fork.c complain here and
according to gcc -E the compiler makes here:
| static void __exit_signal(struct task_struct *$r25)
| {
|  struct signal_struct *sig =3D $r25->signal;
|  bool group_dead =3D thread_group_leader($r25);
=E2=80=A6

and it apparently treating $r25 as a variable name. The assembly appears
correct here:
| ! kernel/exit.c:100:         sighand =3D rcu_dereference_check(tsk->sigha=
nd,
|         lwi     $r8, [$r0 + (928)]      ! _39, $r25_36(D)->sighand

Since the rename here is not intended I would avoid it in case something
funny happens some day.

Sebastian