On Tue, Dec 07, 2021 at 02:28:22PM +0100, Peter Zijlstra wrote:

+static inline void refcount_inc(refcount_t *r)
+{
+	int one = 1;
+
+	asm_volatile_goto (LOCK_PREFIX "xaddl %%eax, %[var]\n\t"
+			   "addl $1, %%eax\n\t"
+			   "je %l[cc_zero]\n\t"
+			   "js %l[cc_error]"
+			   : : [var] "m" (r->refs.counter), "a" (one)
+			   : "memory"
+			   : cc_zero, cc_error);

+       asm_volatile_goto (LOCK_PREFIX "xaddl %[reg], %[var]\n\t"
+                          "addl $1, %[reg]\n\t"
+                          "jz %l[cc_zero]\n\t"
+                          "js %l[cc_error]"
+                          : : [var] "m" (r->refs.counter), [reg] "r" (1)
+                          : "memory"
+                          : cc_zero, cc_error);

Is of course a better implementation, but I'm not sure I actually
understand this code. Afaict: add $1,%[reg], will only set ZF when
%[reg] was -1 such that the result is now 0.

But that's not what the code said; is this GCC going funny in the head
or should I just stop staring at this...