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

Peter Zijlstra
2021-12-07T20:28:40Z

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

> For refcount_inc(), as extracted from alloc_perf_context(), I get:
>
> 4b68: b8 01 00 00 00 mov $0x1,%eax
> 4b6d: f0 0f c1 43 28 lock xadd %eax,0x28(%rbx)
> 4b72: 85 c0 test %eax,%eax
> 4b74: 74 1b je 4b91 <alloc_perf_context+0xf1>
> 4b76: 8d 50 01 lea 0x1(%rax),%edx
> 4b79: 09 c2 or %eax,%edx
> 4b7b: 78 20 js 4b9d <alloc_perf_context+0xfd>
>
> the best I can seem to find is: https://godbolt.org/z/ne5o6eEEW

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.