On Tue, Dec 7, 2021 at 5:28 AM Peter Zijlstra [off-list ref] wrote:

+#define refcount_dec_and_test refcount_dec_and_test
+static inline bool refcount_dec_and_test(refcount_t *r)
+{
+       asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
+                          "jz %l[cc_zero]\n\t"
+                          "jl %l[cc_error]"
+                          : : [var] "m" (r->refs.counter)
+                          : "memory"
+                          : cc_zero, cc_error);
+       return false;
+
+cc_zero:
+       return true;
+
+cc_error:
+       refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
+       return false;
+}

Please don't add broken arch-specific helpers that are useless for
anything else than the broken refcount_t use.

Make it return three return values, call it atomic_dec_and_test_ref()
or something like that, and now people can use it without having to
use a refcount_t.

Nobody really wants two different error cases anyway. The fact that
refcount_warn_saturate() has different cases is only an annoyance.

             Linus