linux-kernel - Re: [PATCH memory-model 2/4] Documentation/litmus-tests: Demonstrate unordered failing cmpxchg

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Date: Wed, 8 May 2024 03:17:15 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	kernel-team@...a.com, mingo@...nel.org, will@...nel.org,
	peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
	dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
	akiyks@...il.com, Frederic Weisbecker <frederic@...nel.org>,
	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Mark Rutland <mark.rutland@....com>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH memory-model 2/4] Documentation/litmus-tests: Demonstrate
 unordered failing cmpxchg

> > Don't the annotations in linux-kernel.def and linux-kernel.bell (like
> > "noreturn") already make this explicit?
> 
> Not that I'm aware. All I can see there is that according to .bell RMW don't
> have an mb mode, but according to .def they do.
> 
> How this mb disappears between parsing the code (.def) and interpreting it
> (.bell) is totally implicit. Including how noreturn affects this
> disappeareance.

IIRC, that's more or less implicit  ;-) in the herd macros of the .def file;
for example,


  (noreturn)

  - atomic_add(V,X) { __atomic_op(X,+,V); }

    Generates a pair of R*[noreturn] and W*[once] events


  (w/ return)

  - atomic_add_return_relaxed(V,X) __atomic_op_return{once}(X,+,V)

    Generates a pair of R*[once] and W*[once] events

  - atomic_add_return_acquire(V,X) __atomic_op_return{acquire}(X,+,V)

    Generates a pair of R*[acquire] and W*[once] events

  - atomic_add_return_release(V,X) __atomic_op_return{acquire}(X,+,V)

    Generates a pair of R*[once] and W*[release] events

  - atomic_add_return(V,X) __atomic_op_return{mb}(X,+,V)

    Generates a pair of R*[once] and W*[once] events plus two F[mb] events


  (possibly failing)

  - atomic_cmpxchg_relaxed(X,V,W) __cmpxchg{once}(X,V,W)

    Generates a pair of R*[once] and W*[once] events if successful;
    a single R*[once] event otherwise.

  - atomic_cmpxchg_acquire(X,V,W) __cmpxchg{acquire}(X,V,W)

    Generates a pair of R*[acquire] and W*[once] events if successful;
    a single R*[once] event otherwise.

  - atomic_cmpxchg_release(X,V,W) __cmpxchg{release}(X,V,W)

    Generates a pair of R*[once] and W*[release] events if successful;
    a single R*[once] event otherwise.

  - atomic_cmpxchg(X,V,W) __cmpxchg{mb}(X,V,W)

    Generates a pair of R*[once] and W*[once] events plus two F[mb] events
    if successful; a single R*[once] event otherwise.


The line

  instructions RMW[{'once,'acquire,'release}]

in the .bell file seems to be effectively redundant (perhaps a LISA backward
-compatibility?): consider

  $ cat rmw.litmus
  C rmw
    
  {}
    
  P0(atomic_t *x)
  {
	int r0;

	r0 = atomic_inc_return_release(x);
  }

  exists (x=1)

Some experiments:

  - Upon removing 'release from "(instructions) RMW"

    $ herd7 -conf linux-kernel.cfg rmw.litmus 
    Test rmw Allowed
    States 1
    [x]=1;
    Ok
    Witnesses
    Positive: 1 Negative: 0
    Condition exists ([x]=1)
    Observation rmw Always 1 0
    Time rmw 0.00
    Hash=3a2dd354c250206d993d31f05f3f595c

  - Upon restoring 'release in RMW and removing it from W

    $ herd7 -conf linux-kernel.cfg rmw.litmus 
    Test rmw Allowed
    States 1
    [x]=1;
    Ok
    Witnesses
    Positive: 1 Negative: 0
    Condition exists ([x]=1)
    Observation rmw Always 1 0
    Time rmw 0.00
    Hash=3a2dd354c250206d993d31f05f3f595c

  - Upon removing 'release from both W and RMW

    $ herd7 -conf linux-kernel.cfg rmw.litmus (herd complains... )
    File "./linux-kernel.bell", line 76, characters 32-39: unbound var: Release

But I'd have to defer to Luc/Jade about herd internals and/or recommended style
on this matter.

  Andrea