Subject: Re: [PATCH RESEND bpf-next 1/1] powerpc64/bpf: Add jit support for load_acquire and store_release

From: Saket Kumar Bhaskar <[email-masked]>
Date: Thu, 24 Jul 2025 15:57:47 +0530

On Thu, Jul 17, 2025 at 08:56:45PM +0000, [email-masked] wrote:
> Puranjay Mohan <[email-masked]> 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@linux-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@linux-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

Hi Puranjay,

Thanks for the patch. I applied the patch and tested it.

Before this patch:

# ./test_progs -a \
  verifier_load_acquire,verifier_store_release,atomics
#11/1    atomics/add:OK
#11/2    atomics/sub:OK
#11/3    atomics/and:OK
#11/4    atomics/or:OK
#11/5    atomics/xor:OK
#11/6    atomics/cmpxchg:OK
#11/7    atomics/xchg:OK
#11      atomics:OK
#528/1   verifier_load_acquire/Clang version < 18, ENABLE_ATOMICS_TESTS not defined, and/or JIT doesn't support load-acquire, use a dummy test:OK
#528     verifier_load_acquire:OK
#565/1   verifier_store_release/Clang version < 18, ENABLE_ATOMICS_TESTS not defined, and/or JIT doesn't support store-release, use a dummy test:OK
#565     verifier_store_release:OK
Summary: 3/9 PASSED, 0 SKIPPED, 0 FAILED

After this patch:

# ./test_progs -a \
  verifier_load_acquire,verifier_store_release,atomics
#11/1    atomics/add:OK
#11/2    atomics/sub:OK
#11/3    atomics/and:OK
#11/4    atomics/or:OK
#11/5    atomics/xor:OK
#11/6    atomics/cmpxchg:OK
#11/7    atomics/xchg:OK
#11      atomics:OK
#529/1   verifier_load_acquire/load-acquire, 8-bit:OK
#529/2   verifier_load_acquire/load-acquire, 8-bit @unpriv:OK
#529/3   verifier_load_acquire/load-acquire, 16-bit:OK
#529/4   verifier_load_acquire/load-acquire, 16-bit @unpriv:OK
#529/5   verifier_load_acquire/load-acquire, 32-bit:OK
#529/6   verifier_load_acquire/load-acquire, 32-bit @unpriv:OK
#529/7   verifier_load_acquire/load-acquire, 64-bit:OK
#529/8   verifier_load_acquire/load-acquire, 64-bit @unpriv:OK
#529/9   verifier_load_acquire/load-acquire with uninitialized src_reg:OK
#529/10  verifier_load_acquire/load-acquire with uninitialized src_reg @unpriv:OK
#529/11  verifier_load_acquire/load-acquire with non-pointer src_reg:OK
#529/12  verifier_load_acquire/load-acquire with non-pointer src_reg @unpriv:OK
#529/13  verifier_load_acquire/misaligned load-acquire:OK
#529/14  verifier_load_acquire/misaligned load-acquire @unpriv:OK
#529/15  verifier_load_acquire/load-acquire from ctx pointer:OK
#529/16  verifier_load_acquire/load-acquire from ctx pointer @unpriv:OK
#529/17  verifier_load_acquire/load-acquire with invalid register R15:OK
#529/18  verifier_load_acquire/load-acquire with invalid register R15 @unpriv:OK
#529/19  verifier_load_acquire/load-acquire from pkt pointer:OK
#529/20  verifier_load_acquire/load-acquire from flow_keys pointer:OK
#529/21  verifier_load_acquire/load-acquire from sock pointer:OK
#529     verifier_load_acquire:OK
#566/1   verifier_store_release/store-release, 8-bit:OK
#566/2   verifier_store_release/store-release, 8-bit @unpriv:OK
#566/3   verifier_store_release/store-release, 16-bit:OK
#566/4   verifier_store_release/store-release, 16-bit @unpriv:OK
#566/5   verifier_store_release/store-release, 32-bit:OK
#566/6   verifier_store_release/store-release, 32-bit @unpriv:OK
#566/7   verifier_store_release/store-release, 64-bit:OK
#566/8   verifier_store_release/store-release, 64-bit @unpriv:OK
#566/9   verifier_store_release/store-release with uninitialized src_reg:OK
#566/10  verifier_store_release/store-release with uninitialized src_reg @unpriv:OK
#566/11  verifier_store_release/store-release with uninitialized dst_reg:OK
#566/12  verifier_store_release/store-release with uninitialized dst_reg @unpriv:OK
#566/13  verifier_store_release/store-release with non-pointer dst_reg:OK
#566/14  verifier_store_release/store-release with non-pointer dst_reg @unpriv:OK
#566/15  verifier_store_release/misaligned store-release:OK
#566/16  verifier_store_release/misaligned store-release @unpriv:OK
#566/17  verifier_store_release/store-release to ctx pointer:OK
#566/18  verifier_store_release/store-release to ctx pointer @unpriv:OK
#566/19  verifier_store_release/store-release, leak pointer to stack:OK
#566/20  verifier_store_release/store-release, leak pointer to stack @unpriv:OK
#566/21  verifier_store_release/store-release, leak pointer to map:OK
#566/22  verifier_store_release/store-release, leak pointer to map @unpriv:OK
#566/23  verifier_store_release/store-release with invalid register R15:OK
#566/24  verifier_store_release/store-release with invalid register R15 @unpriv:OK
#566/25  verifier_store_release/store-release to pkt pointer:OK
#566/26  verifier_store_release/store-release to flow_keys pointer:OK
#566/27  verifier_store_release/store-release to sock pointer:OK
#566     verifier_store_release:OK
Summary: 3/55 PASSED, 0 SKIPPED, 0 FAILED

Tested-by: Saket Kumar Bhaskar <[email-masked]>

Regards,
Saket