linux-kernel - Re: [PATCH] block: switch to atomic

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Message-ID: <20211207202831.GA18361@worktop.programming.kicks-ass.net>
Date:   Tue, 7 Dec 2021 21:28:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        keescook@...omium.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: switch to atomic_t for request references

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.