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Message-ID: <8cb4e810-da6c-4d8f-b608-5bdba56da2bd@gmail.com>
Date: Sat, 21 Sep 2024 11:44:32 +0900
From: Akira Yokosawa <akiyks@...il.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, paulmck@...nel.org
Cc: stern@...land.harvard.edu, parri.andrea@...il.com, will@...nel.org,
peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
dlustig@...dia.com, joel@...lfernandes.org, urezki@...il.com,
quic_neeraju@...cinc.com, frederic@...nel.org, linux-kernel@...r.kernel.org,
lkmm@...ts.linux.dev, hernan.poncedeleon@...weicloud.com,
Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH v3 4/5] tools/memory-model: Switch to softcoded herd7 tags
On Thu, 19 Sep 2024 15:06:33 +0200, Jonas Oberhauser wrote:
> A new version of Herd7 provides a -lkmmv1 switch which overrides the old herd7
Why -lkmmv1?
You mean current (unversioned) LKMM has to be called v0 ???
My preference is to call current one as v1 and your new version as v2.
Either way,
Reviewed-by: Akira Yokosawa <akiyks@...il.com>
Please find a few more comments inline below.
> behavior of simply ignoring any softcoded tags in the .def and .bell files. We
> port LKMM to this version of Herd7 by providing the switch in linux-kernel.cfg
> and reporting an error if the LKMM is used without this switch.
>
> To preserve the semantics of LKMM, we also softcode the Noreturn tag on atomic
> RMW which do not return a value and define atomic_add_unless with an Mb tag in
> linux-kernel.def.
>
> We update the herd-representation.txt accordingly and clarify some of the
> resulting combinations.
>
> We also add a litmus test for atomic_add_unless which uncovered a bug in early
> iterations of the Herd7 patch that implements the new switch.
>
> (To be) Signed-off-by: Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
> Signed-off by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
> ---
> .../Documentation/herd-representation.txt | 27 ++++++++++---------
> tools/memory-model/linux-kernel.bell | 3 +++
> tools/memory-model/linux-kernel.cfg | 1 +
> tools/memory-model/linux-kernel.def | 18 +++++++------
> .../litmus-tests/add-unless-mb.litmus | 27 +++++++++++++++++++
> 5 files changed, 56 insertions(+), 20 deletions(-)
> create mode 100644 tools/memory-model/litmus-tests/add-unless-mb.litmus
>
[...]
> diff --git a/tools/memory-model/linux-kernel.def b/tools/memory-model/linux-kernel.def
> index a12b96c547b7..4281572732bd 100644
> --- a/tools/memory-model/linux-kernel.def
> +++ b/tools/memory-model/linux-kernel.def
> @@ -63,14 +63,14 @@ atomic_set(X,V) { WRITE_ONCE(*X,V); }
> atomic_read_acquire(X) smp_load_acquire(X)
> atomic_set_release(X,V) { smp_store_release(X,V); }
>
> -atomic_add(V,X) { __atomic_op(X,+,V); }
> -atomic_sub(V,X) { __atomic_op(X,-,V); }
> -atomic_and(V,X) { __atomic_op(X,&,V); }
> -atomic_or(V,X) { __atomic_op(X,|,V); }
> -atomic_xor(V,X) { __atomic_op(X,^,V); }
> -atomic_inc(X) { __atomic_op(X,+,1); }
> -atomic_dec(X) { __atomic_op(X,-,1); }
> -atomic_andnot(V,X) { __atomic_op(X,&~,V); }
> +atomic_add(V,X) { __atomic_op{noreturn}(X,+,V); }
> +atomic_sub(V,X) { __atomic_op{noreturn}(X,-,V); }
> +atomic_and(V,X) { __atomic_op{noreturn}(X,&,V); }
> +atomic_or(V,X) { __atomic_op{noreturn}(X,|,V); }
> +atomic_xor(V,X) { __atomic_op{noreturn}(X,^,V); }
> +atomic_inc(X) { __atomic_op{noreturn}(X,+,1); }
> +atomic_dec(X) { __atomic_op{noreturn}(X,-,1); }
> +atomic_andnot(V,X) { __atomic_op{noreturn}(X,&~,V); }
>
> atomic_add_return(V,X) __atomic_op_return{mb}(X,+,V)
> atomic_add_return_relaxed(V,X) __atomic_op_return{once}(X,+,V)
> @@ -144,3 +144,5 @@ atomic_fetch_andnot(V,X) __atomic_fetch_op{mb}(X,&~,V)
> atomic_fetch_andnot_acquire(V,X) __atomic_fetch_op{acquire}(X,&~,V)
> atomic_fetch_andnot_release(V,X) __atomic_fetch_op{release}(X,&~,V)
> atomic_fetch_andnot_relaxed(V,X) __atomic_fetch_op{once}(X,&~,V)
> +
> +atomic_add_unless(X,V,W) __atomic_add_unless{mb}(X,V,W)
> \ No newline at end of file
Please fix this warning.
> diff --git a/tools/memory-model/litmus-tests/add-unless-mb.litmus b/tools/memory-model/litmus-tests/add-unless-mb.litmus
> new file mode 100644
> index 000000000000..72f76ff3f59d
> --- /dev/null
> +++ b/tools/memory-model/litmus-tests/add-unless-mb.litmus
> @@ -0,0 +1,27 @@
> +C add_unless_mb
> +
> +(*
> + * Result: Never
> + *
> + * This litmus test demonstrates that a successful atomic_add_unless
> + * acts as a full memory barrier, ensuring that *x=1 propagates to P1
> + * before P1 executes *x=2.
> + *)
> +
> +{}
> +
> +P0(atomic_t *x, atomic_t *y, atomic_t *z)
> +{
> + WRITE_ONCE(*x, 1);
> + int r0 = atomic_add_unless(z,1,5);
> + WRITE_ONCE(*y, 1);
> +}
> +
> +P1(atomic_t *x, atomic_t *y)
> +{
> + int r0 = READ_ONCE(*y);
> + if (r0 == 1)
> + WRITE_ONCE(*x, 2);
> +}
> +
> +exists (1:r0=1 /\ x=1)
This litmus test is not compatible with klitmus7, which is much
stricter than herd7's C parser.
You can have only int or int* variables in the exists clause.
Register variables need their declarations at the top of each Pn()
(classic C).
See below for klitmus7 ready code.
And tools/memory-model/litmus-tests/README need to mention this
litmus test.
Thanks, Akira
---------------------------------------------
P0(int *x, int *y, atomic_t *z)
{
int r0;
WRITE_ONCE(*x, 1);
r0 = atomic_add_unless(z,1,5);
WRITE_ONCE(*y, 1);
}
P1(int *x, int *y)
{
int r0;
r0 = READ_ONCE(*y);
if (r0 == 1)
WRITE_ONCE(*x, 2);
}
exists (1:r0=1 /\ x=1)
---------------------------------------------
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