linux-kernel - Re: [PATCH net-next v3 01/17] asm: simd context helper API

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Message-Id: <61B4DFD6-9BB5-4B81-BD0E-ACF112EA8AF7@amacapital.net>
Date:   Sat, 15 Sep 2018 12:58:21 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     kevin@...rana.org, Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Samuel Neves <sneves@....uc.pt>, linux-arch@...r.kernel.org
Subject: Re: [PATCH net-next v3 01/17] asm: simd context helper API


> On Sep 13, 2018, at 6:52 AM, Jason A. Donenfeld <Jason@...c4.com> wrote:
> 
>> On Thu, Sep 13, 2018 at 7:03 AM Kevin Easton <kevin@...rana.org> wrote:
>> Yes.  It's also how most get/put APIs already work in the kernel, eg
>> kref_get/put (mostly because they tend to be 'getting/putting' an
>> already-initialized object, though).
> 
> Right; in this case the object wouldn't be initialized yet, which
> might defeat the purpose, since one advantage of the & way you
> mentioned is _put modifies the context.
> 
> Andy - any opinions on this? The tl;dr is:
> 
> 1) what we have now:
> 
> simd_context_t simd_context = simd_get();
> for (item in items) {
>    do_something(item);
>    simd_context = simd_relax(simd_context);
> }
> simd_put();
> 
> 2) what kevin is proposing:
> 
> simd_context_t simd_context;
> 
> simd_get(&simd_context);
> for (item in items) {
>    do_something(item);
>    simd_relax(&simd_context);
> }
> simd_put(&simd_context);
> 
> I can see pros and cons of each approach.

I have no strong opinion, except that, if you paint the bikeshed “pass by value”, then simd_get and simd_relax should probably have warn_unused_result set.