Messages in this thread | | | Date | Wed, 8 Dec 2021 18:07:36 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] block: switch to atomic_t for request references |
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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.
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