[PATCH] floppy: Avoid unsequenced accesses to jiffies in show_floppy()

[PATCH] floppy: Avoid unsequenced accesses to jiffies in show_floppy()

[Nathan Chancellor]

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

  drivers/block/floppy.c:1839:30: error: unsequenced accesses to volatile qualified 'jiffies' [-Werror,-Wunsequenced]
   1839 |                 jiffies, interruptjiffies, jiffies - interruptjiffies,
        |                 ~~~~~~~                    ^

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.

Add a local variable to hold the value of jiffies before the function
call so that there are no unsequenced side effects, clearing up the
warning.

Cc: stable@vger.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/2198
Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/block/floppy.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index c677d783811f..f6339d779be9 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -1830,13 +1830,14 @@ static void reset_fdc(void)
 
 static void show_floppy(int fdc)
 {
+	unsigned long j = jiffies;
 	int i;
 
 	pr_info("\n");
 	pr_info("floppy driver state\n");
 	pr_info("-------------------\n");
 	pr_info("now=%lu last interrupt=%lu diff=%lu last called handler=%ps\n",
-		jiffies, interruptjiffies, jiffies - interruptjiffies,
+		j, interruptjiffies, j - interruptjiffies,
 		lasthandler);
 
 	pr_info("timeout_message=%s\n", timeout_message);

---
base-commit: ab43ec385580b8f8abf72d70fa3acbf4e4f098a5
change-id: 20261114-floppy-wunsequenced-8b739d08b849

Best regards,
--  
Cheers,
Nathan