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From: Akira Yokosawa <akiyks@gmail.com>
To: Elad Lahav <e2lahav@gmail.com>
Cc: perfbook@vger.kernel.org
Subject: Re: Ordering and conditionals
Date: Thu, 12 Jan 2023 07:20:19 +0900	[thread overview]
Message-ID: <fd3c9cc6-12d4-6d02-0e32-9e6c20deb342@gmail.com> (raw)
In-Reply-To: <CAJbg=FX8rBFRqO-3bhE863OepcdShoyGg0tVeedVR2=tfjwcDQ@mail.gmail.com>

Hi Elad,

On Wed, 11 Jan 2023 11:09:14 -0500, Elad Lahav wrote:
> Hello all,
> 
> Here's an interesting (hopefully!) question I came across. Consider
> the following code:
> 
> int a = 0;
> int b = 0;
> 
> void
> writer()
> {
>     a = 1;
>     write_barrier();
>     b = 2;
> }
> 
> int
> reader1()
> {
>     for (;;) {
>         if (b == 2) {
>             return a;
>         }
> 
>         yield();
>     }
> }
> 
> int
> reader2()
> {
>     while (b == 2) {
>         yield();
>     }
> 
>     return a;
> }
> 
> The barrier issued by the writer ensures that if any reader observes b
> == 2 then it must also observe a == 1. However, that is only true if
> both the compiler and the processor agree that a is loaded after b.
> 
> Consider the two reader functions. These are pretty much the same from
> C's point of view, but while the dependency between b and a is clear
> in the first case it is less so in the second. I can see a compiler
> loading a before the loop in reader2(), but less so in the case of
> reader1(). Perhaps that's just the way I interpret the code and a
> compiler would treat these as exactly the same.
> 
> Let's assume then that you change the code to do something like return
> READ_ONCE(a) in both cases, as the book recommends. Does it change
> anything for the compiler? Does it change anything for the processor,
> which can reorder the reads? Anecdotally, it seems that a read barrier
> helps with reader2(), but I would not expect it to be needed for
> reader1().
> 
> Thoughts?

I think Section 15.2.5 "Control Dependencies" can answer your question.

TLDR, control dependencies are good for load-to-store ordering.
You are talking about load-to-load ordering and control dependencies
don't suffice.

        Thanks, Akira

> 
> --Elad

     prev parent reply	other threads:[~2023-01-11 22:20 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-11 16:09 Ordering and conditionals Elad Lahav
2023-01-11 22:20 ` Akira Yokosawa [this message]

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