[PATCH] powerpc/xmon: Avoid unsequenced accesses to tsk in show_task()

[PATCH] powerpc/xmon: Avoid unsequenced accesses to tsk in show_task()

[Nathan Chancellor]

After a recent improvement in clang to warn on unsequenced accesses to
volatile variables [1], an instance appears in show_task(), which can
break the build with CONFIG_PPC_WERROR=y / CONFIG_WERROR=y / W=e:

  arch/powerpc/xmon/xmon.c:3262:3: error: unsequenced accesses to volatile qualified 'tsk' [-Werror,-Wunsequenced]
   3261 |         printf("%16px %16lx %16px %6d %6d %c %2d %s\n", tsk,
        |                                                         ~~~
   3262 |                 tsk->thread.ksp, tsk->thread.regs,
        |                 ^

Per C11 5.1.2.3.2, accessing a volatile object is a side effect. Per C11
6.5.2, unsequenced side effects on the same scalar object are undefined
behavior. Per C11 6.5.2.2.10/12, the evaluation of arguments in a
function call is indeterminately sequenced, meaning any side effects in
the arguments are unsequenced.

tsk was made volatile in commit a2305e3de819 ("powerpc: mark local
variables around longjmp as volatile") to ensure its value in
show_tasks() was properly restored after returning from a potential
longjmp(). Aside from that, the value is not being modified through
other means, so rename the volatile parameter and add a local variable
named tsk to hold its value throughout show_task(), clearing up the
warning.

Cc: stable@vger.kernel.org
Fixes: a2305e3de819 ("powerpc: mark local variables around longjmp as volatile")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2199
Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/powerpc/xmon/xmon.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index cb3a3244ae6f..90a8d7a8f42d 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -3240,8 +3240,9 @@ memzcan(void)
 		printf("%.8lx\n", a - mskip);
 }
 
-static void show_task(struct task_struct *volatile tsk)
+static void show_task(struct task_struct *volatile vol_tsk)
 {
+	struct task_struct *tsk = vol_tsk;
 	unsigned int p_state = READ_ONCE(tsk->__state);
 	char state;
 

---
base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
base-tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
change-id: 20261115-xmon-wunsequenced-120aeacf7636

Best regards,
--  
Cheers,
Nathan