Re: [PATCH] block: switch to atomic_t for request references

Peter Zijlstra
2021-12-07T15:51:56Z

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...