From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Puranjay Mohan <puranjay@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
Jiri Olsa <jolsa@kernel.org>,
"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Nicholas Piggin <npiggin@gmail.com>,
Aneesh Kumar K.V <aneesh.kumar@kernel.org>,
Hari Bathini <hbathini@linux.ibm.com>,
"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"paulmck@kernel.org" <paulmck@kernel.org>
Cc: "puranjay12@gmail.com" <puranjay12@gmail.com>
Subject: Re: [PATCH bpf v2] powerpc/bpf: enforce full ordering for ATOMIC operations with BPF_FETCH
Date: Mon, 13 May 2024 06:18:04 +0000 [thread overview]
Message-ID: <73170d74-332c-45d4-883a-f4cda12f3542@csgroup.eu> (raw)
In-Reply-To: <20240508115404.74823-1-puranjay@kernel.org>
Le 08/05/2024 à 13:54, Puranjay Mohan a écrit :
> [Vous ne recevez pas souvent de courriers de puranjay@kernel.org. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
>
> The Linux Kernel Memory Model [1][2] requires RMW operations that have a
> return value to be fully ordered.
>
> BPF atomic operations with BPF_FETCH (including BPF_XCHG and
> BPF_CMPXCHG) return a value back so they need to be JITed to fully
> ordered operations. POWERPC currently emits relaxed operations for
> these.
>
> We can show this by running the following litmus-test:
>
> PPC SB+atomic_add+fetch
>
> {
> 0:r0=x; (* dst reg assuming offset is 0 *)
> 0:r1=2; (* src reg *)
> 0:r2=1;
> 0:r4=y; (* P0 writes to this, P1 reads this *)
> 0:r5=z; (* P1 writes to this, P0 reads this *)
> 0:r6=0;
>
> 1:r2=1;
> 1:r4=y;
> 1:r5=z;
> }
>
> P0 | P1 ;
> stw r2, 0(r4) | stw r2,0(r5) ;
> | ;
> loop:lwarx r3, r6, r0 | ;
> mr r8, r3 | ;
> add r3, r3, r1 | sync ;
> stwcx. r3, r6, r0 | ;
> bne loop | ;
> mr r1, r8 | ;
> | ;
> lwa r7, 0(r5) | lwa r7,0(r4) ;
>
> ~exists(0:r7=0 /\ 1:r7=0)
>
> Witnesses
> Positive: 9 Negative: 3
> Condition ~exists (0:r7=0 /\ 1:r7=0)
> Observation SB+atomic_add+fetch Sometimes 3 9
>
> This test shows that the older store in P0 is reordered with a newer
> load to a different address. Although there is a RMW operation with
> fetch between them. Adding a sync before and after RMW fixes the issue:
>
> Witnesses
> Positive: 9 Negative: 0
> Condition ~exists (0:r7=0 /\ 1:r7=0)
> Observation SB+atomic_add+fetch Never 0 9
>
> [1] https://www.kernel.org/doc/Documentation/memory-barriers.txt
> [2] https://www.kernel.org/doc/Documentation/atomic_t.txt
>
> Fixes: 65112709115f ("powerpc/bpf/64: add support for BPF_ATOMIC bitwise operations")
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
> Changes in v1 -> v2:
> v1: https://lore.kernel.org/all/20240507175439.119467-1-puranjay@kernel.org/
> - Don't emit `sync` for non-SMP kernels as that adds unessential overhead.
> ---
> arch/powerpc/net/bpf_jit_comp32.c | 12 ++++++++++++
> arch/powerpc/net/bpf_jit_comp64.c | 12 ++++++++++++
> 2 files changed, 24 insertions(+)
>
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index 2f39c50ca729..0318b83f2e6a 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -853,6 +853,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
> /* Get offset into TMP_REG */
> EMIT(PPC_RAW_LI(tmp_reg, off));
> tmp_idx = ctx->idx * 4;
> + /*
> + * Enforce full ordering for operations with BPF_FETCH by emitting a 'sync'
> + * before and after the operation.
> + *
> + * This is a requirement in the Linux Kernel Memory Model.
> + * See __cmpxchg_u64() in asm/cmpxchg.h as an example.
You mean __cmpxchg_u32() ?
> + */
> + if (imm & BPF_FETCH && IS_ENABLED(CONFIG_SMP))
Please enclose imm & BPF_FETCH inside a () to improve readability.
> + EMIT(PPC_RAW_SYNC());
Do you really want to do it inside the loop at each try ? Because when
stwcx. fails it jumps back at tmp_idx which is your new sync now.
> /* load value from memory into r0 */
> EMIT(PPC_RAW_LWARX(_R0, tmp_reg, dst_reg, 0));
>
> @@ -905,6 +914,9 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
>
> /* For the BPF_FETCH variant, get old data into src_reg */
> if (imm & BPF_FETCH) {
> + /* Emit 'sync' to enforce full ordering */
> + if (IS_ENABLED(CONFIG_SMP))
> + EMIT(PPC_RAW_SYNC());
> EMIT(PPC_RAW_MR(ret_reg, ax_reg));
> if (!fp->aux->verifier_zext)
> EMIT(PPC_RAW_LI(ret_reg - 1, 0)); /* higher 32-bit */
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index 79f23974a320..9a077f8acf7b 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -804,6 +804,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
> /* Get offset into TMP_REG_1 */
> EMIT(PPC_RAW_LI(tmp1_reg, off));
> tmp_idx = ctx->idx * 4;
> + /*
> + * Enforce full ordering for operations with BPF_FETCH by emitting a 'sync'
> + * before and after the operation.
> + *
> + * This is a requirement in the Linux Kernel Memory Model.
> + * See __cmpxchg_u64() in asm/cmpxchg.h as an example.
> + */
> + if (imm & BPF_FETCH && IS_ENABLED(CONFIG_SMP))
Please enclose imm & BPF_FETCH inside a () to improve readability.
> + EMIT(PPC_RAW_SYNC());
Same, the sync should be outside the loop I guess.
> /* load value from memory into TMP_REG_2 */
> if (size == BPF_DW)
> EMIT(PPC_RAW_LDARX(tmp2_reg, tmp1_reg, dst_reg, 0));
> @@ -865,6 +874,9 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
> PPC_BCC_SHORT(COND_NE, tmp_idx);
>
> if (imm & BPF_FETCH) {
> + /* Emit 'sync' to enforce full ordering */
> + if (IS_ENABLED(CONFIG_SMP))
> + EMIT(PPC_RAW_SYNC());
> EMIT(PPC_RAW_MR(ret_reg, _R0));
> /*
> * Skip unnecessary zero-extension for 32-bit cmpxchg.
> --
> 2.42.0
>
prev parent reply other threads:[~2024-05-13 6:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-08 11:54 [PATCH bpf v2] powerpc/bpf: enforce full ordering for ATOMIC operations with BPF_FETCH Puranjay Mohan
2024-05-08 14:25 ` Paul E. McKenney
2024-05-08 17:02 ` Naveen N Rao
2024-05-13 6:18 ` Christophe Leroy [this message]
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