linux-kernel - Re: [PATCH RESEND bpf-next 1/1] powerpc64/bpf: Add jit support for load_acquire and store

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Message-ID: <mb61pfreuy1rm.fsf@kernel.org>
Date: Thu, 17 Jul 2025 20:56:45 +0000
From: <puranjay@...nel.org>
To: Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman
 <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Christophe
 Leroy <christophe.leroy@...roup.eu>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Eduard
 Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
 <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, KP
 Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Hari Bathini
 <hbathini@...ux.ibm.com>, Naveen N Rao <naveen@...nel.org>, Mykola Lysenko
 <mykolal@...com>, Peilin Ye <yepeilin@...gle.com>, Kumar Kartikeya Dwivedi
 <memxor@...il.com>
Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org, "Paul E . McKenney" <paulmck@...nel.org>,
 lkmm@...ts.linux.dev
Subject: Re: [PATCH RESEND bpf-next 1/1] powerpc64/bpf: Add jit support for
 load_acquire and store_release

Puranjay Mohan <puranjay@...nel.org> writes:

Somehow the cover letter for this patch was missed, adding it here:

To test the functionality of these special instructions, a tool called
blitmus[0] was used to convert the following baseline litmus test[1] to bpf
programs:

 C MP+poonceonces

 (*
  * Result: Sometimes
  *
  * Can the counter-intuitive message-passing outcome be prevented with
  * no ordering at all?
  *)

 {}

 P0(int *buf, int *flag)
 {
         WRITE_ONCE(*buf, 1);
         WRITE_ONCE(*flag, 1);
 }

 P1(int *buf, int *flag)
 {
         int r0;
         int r1;

         r0 = READ_ONCE(*flag);
         r1 = READ_ONCE(*buf);
 }

 exists (1:r0=1 /\ 1:r1=0) (* Bad outcome. *)

Running the generated bpf program shows that the bad outcome is possible on
powerpc:

 [fedora@...ux-kernel blitmus]$ sudo ./mp_poonceonces
 Starting litmus test with configuration:
   Test: MP+poonceonces
   Iterations: 4100

 Test MP+poonceonces Allowed
 Histogram (4 states)
 21548375 :>1:r0=0; 1:r1=0;
 301187   :>1:r0=0; 1:r1=1;
 337147   *>1:r0=1; 1:r1=0;
 18813291 :>1:r0=1; 1:r1=1;
 Ok

 Witnesses
 Positive: 337147, Negative: 40662853
 Condition exists (1:r0=1 /\ 1:r1=0) is validated
 Observation MP+poonceonces Sometimes 337147 40662853
 Time MP+poonceonces 13.48

 Thu Jul 17 18:12:51 UTC

Now the second write and the first read is converted to store_release and
load_acquire and it gives us the following litmus test[2]

 C MP+pooncerelease+poacquireonce

 (*
  * Result: Never
  *
  * This litmus test demonstrates that smp_store_release() and
  * smp_load_acquire() provide sufficient ordering for the message-passing
  * pattern.
  *)

 {}

 P0(int *buf, int *flag)
 {
         WRITE_ONCE(*buf, 1);
         smp_store_release(flag, 1);
 }

 P1(int *buf, int *flag)
 {
         int r0;
         int r1;

         r0 = smp_load_acquire(flag);
         r1 = READ_ONCE(*buf);
 }

 exists (1:r0=1 /\ 1:r1=0) (* Bad outcome. *)

Running the generated bpf program shows that the bad outcome is *not* possible
on powerpc with the implementation in this patch:

 [fedora@...ux-kernel blitmus]$ sudo ./mp_pooncerelease_poacquireonce
 Starting litmus test with configuration:
   Test: MP+pooncerelease+poacquireonce
   Iterations: 4100

 Test MP+pooncerelease+poacquireonce Allowed
 Histogram (3 states)
 21036021 :>1:r0=0; 1:r1=0;
 14488694 :>1:r0=0; 1:r1=1;
 5475285  :>1:r0=1; 1:r1=1;
 No

 Witnesses
 Positive: 0, Negative: 41000000
 Condition exists (1:r0=1 /\ 1:r1=0) is NOT validated
 Observation MP+pooncerelease+poacquireonce Never 0 41000000
 Time MP+pooncerelease+poacquireonce 13.74

 Thu Jul 17 18:13:40 UTC

[0] https://github.com/puranjaymohan/blitmus
[1] https://github.com/puranjaymohan/blitmus/blob/main/litmus_tests/MP%2Bpoonceonces.litmus
[2] https://github.com/puranjaymohan/blitmus/blob/main/litmus_tests/MP%2Bpooncerelease%2Bpoacquireonce.litmus