percpu: Fix self-assignment of __old in raw_cpu_generic_try_cmpxchg()

After commit c5c0ba953b8c ("percpu: Add {raw,this}_cpu_try_cmpxchg()"),
clang built ARCH=arm and ARCH=arm64 kernels with CONFIG_INIT_STACK_NONE
started panicking on boot in alloc_vmap_area():

  [    0.000000] kernel BUG at mm/vmalloc.c:1638!
  [    0.000000] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
  [    0.000000] Modules linked in:
  [    0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.4.0-rc2-ARCH+ #1
  [    0.000000] Hardware name: linux,dummy-virt (DT)
  [    0.000000] pstate: 200000c9 (nzCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  [    0.000000] pc : alloc_vmap_area+0x7ec/0x7f8
  [    0.000000] lr : alloc_vmap_area+0x7e8/0x7f8

Compiling mm/vmalloc.c with W=2 reveals an instance of -Wshadow, which
helps uncover that through macro expansion, '__old = *(ovalp)' in
raw_cpu_generic_try_cmpxchg() can become '__old = *(&__old)' through
raw_cpu_generic_cmpxchg(), which results in garbage being assigned to
the inner __old and the cmpxchg not working properly.

Add an extra underscore to __old in raw_cpu_generic_try_cmpxchg() so
that there is no more self-assignment, which resolves the panics.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1868
Fixes: c5c0ba953b8c ("percpu: Add {raw,this}_cpu_try_cmpxchg()")
Debugged-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230607-fix-shadowing-in-raw_cpu_generic_try_cmpxchg-v1-1-8f0a3d930d43@kernel.org

diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
index 68c410e..94cbd50 100644 (file)
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -101,9 +101,9 @@ do {                                                                        \
 #define raw_cpu_generic_try_cmpxchg(pcp, ovalp, nval)                  \
 ({                                                                     \
        typeof(pcp) *__p = raw_cpu_ptr(&(pcp));                         \
-       typeof(pcp) __val = *__p, __old = *(ovalp);                     \
+       typeof(pcp) __val = *__p, ___old = *(ovalp);                    \
        bool __ret;                                                     \
-       if (__val == __old) {                                           \
+       if (__val == ___old) {                                          \
                *__p = nval;                                            \
                __ret = true;                                           \
        } else {                                                        \