linux-kernel - Re: [PATCH] docs/lkmm: Correct ->prop example with additional rfe link

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Message-ID: <20190728020837.GA26905@tardis>
Date:   Sun, 28 Jul 2019 10:08:37 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>, linux-arch@...r.kernel.org,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] docs/lkmm: Correct ->prop example with additional rfe
 link

Hi Joel,

On Sat, Jul 27, 2019 at 08:00:31PM -0400, Joel Fernandes (Google) wrote:
> This lkmm example should describe an additional rfe link between P1's
> store to y and P2's load of y, which should be critical to establishing
> the ordering resulting in the ->prop ordering on P0. IOW, there are 2 rfe
> links, not one.
> 
> Correct these in the docs to make the ->prop ordering in P0 more clear.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  tools/memory-model/Documentation/explanation.txt | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> index 68caa9a976d0..6c0dfaac7f04 100644
> --- a/tools/memory-model/Documentation/explanation.txt
> +++ b/tools/memory-model/Documentation/explanation.txt
> @@ -1302,8 +1302,8 @@ followed by an arbitrary number of cumul-fence links, ending with an
>  rfe link.  You can concoct more exotic examples, containing more than
>  one fence, although this quickly leads to diminishing returns in terms
>  of complexity.  For instance, here's an example containing a coe link
> -followed by two fences and an rfe link, utilizing the fact that
> -release fences are A-cumulative:
> +followed by a fence, an rfe link, another fence and and a final rfe link,
> +utilizing the fact that release fences are A-cumulative:
>  

This part looks good to me.

>  	int x, y, z;
>  
> @@ -1334,11 +1334,13 @@ If x = 2, r0 = 1, and r2 = 1 after this code runs then there is a prop
>  link from P0's store to its load.  This is because P0's store gets
>  overwritten by P1's store since x = 2 at the end (a coe link), the
>  smp_wmb() ensures that P1's store to x propagates to P2 before the
> -store to y does (the first fence), the store to y propagates to P2
> -before P2's load and store execute, P2's smp_store_release()
> -guarantees that the stores to x and y both propagate to P0 before the
> -store to z does (the second fence), and P0's load executes after the
> -store to z has propagated to P0 (an rfe link).
> +store to y does (the first fence), P2's store to y happens before P2's
> +load of y (rfe link), P2's smp_store_release() ensures that P2's load
> +of y executes before P2's store of z (second fence), which also would
> +imply that stores to x and y happen before the smp_store_release(), which

I think it's more accurate to say:

"imply that stores to x and y progagates to P2 before the
smp_store_release()"

, because by definition the propagation ordering that
smp_store_release() guarantees only works with stores that already
propagated to the CPU executing it, not the stores that execute/happen
before.

With that, feel free to add:

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> +means that P2's smp_store_release() will propagate stores to x and y to all
> +CPUs before the store to z does (A-cumulative property of this fence).
> +Finally P0's load executes after store to z has propagated to P0 (rfe link).
>  
>  In summary, the fact that the hb relation links memory access events
>  in the order they execute means that it must not have cycles.  This
> -- 
> 2.22.0.709.g102302147b-goog
> 

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