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

Peter Zijlstra
2021-12-08T17:07:45Z

On Tue, Dec 07, 2021 at 03:23:02PM -0800, Linus Torvalds wrote:
> On Tue, Dec 7, 2021 at 12:28 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Argh.. __atomic_add_fetch() != __atomic_fetch_add(); much confusion for
> > GCC having both. With the right primitive it becomes:
> >
> > movl $1, %eax
> > lock xaddl %eax, (%rdi)
> > testl %eax, %eax
> > je .L5
> > js .L6
> >
> > Which makes a whole lot more sense.
>
> Note that the above misses the case where the old value was MAX_INT
> and the result now became negative.
>
> That isn't a _problem_, of course. I think it's fine. But if you cared
> about it, you'd have to do something like

Hm....

> But if you don't care about the MAX_INT overflow and make the overflow
> boundary be the next increment, then just make it be one error case:
>
> > movl $1, %eax
> > lock xaddl %eax, (%rdi)
> > testl %eax, %eax
> > jle .L5
>
> and then (if you absolutely have to distinguish them) you can test eax
> again in the slow path.

Suppose:

inc(): overflow when old value is negative or zero
dec(): overflow when new value is negative or zero

That gives:

inc(INT_MAX) is allowed
dec(INT_MIN) is allowed

IOW, the effective range becomes: [1..INT_MIN], which is a bit
counter-intuitive, but then so is most of this stuff.

Therefore can write this like:

#define atomic_inc_ofl(v, label)
do {
int old = atomic_fetch_inc(v);
if (unlikely(old <= 0))
goto label;
} while (0)

#define atomic_dec_ofl(v, label)
do {
int new = atomic_dec_return(v);
if (unlikely(new <= 0))
goto label;
} while (0)

#define atomic_dec_and_test_ofl(v, label)
({
bool ret = false;
int new = atomic_dec_return(&r->refs);
if (unlikely(new < 0))
goto label;
if (unlikely(new == 0)
ret = true;
ret;
})

For a consistent set of primitives, right?

Which already gives better code-gen than we have today.

But that then also means we can write dec_ofl as:

lock decl %[var]
jle %l1

and dec_and_test_ofl() like:

lock decl %[var]
jl %l2
je %l[__zero]

Lemme finisht the patches and send that out after dinner.