Re: [PATCH bpf v3] powerpc/bpf: enforce full ordering for ATOMIC operations with BPF_FETCH

Christophe Leroy
2024-05-13T10:06:08Z

Le 13/05/2024 à 12:02, Puranjay Mohan a écrit :
> 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>
> Acked-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
> Changes in v2 -> v3:
> v2: https://lore.kernel.org/all/20240508115404.74823-1-puranjay@kernel.org/
> - Emit the sync outside the loop so it doesn't get executed everytime.
> - Minor coding style changes.
>
> 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..35f64dcfa68e 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -852,6 +852,15 @@ 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));
> + /*
> + * 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_u32() in asm/cmpxchg.h as an example.
> + */
> + if ((imm & BPF_FETCH) && IS_ENABLED(CONFIG_SMP))
> + EMIT(PPC_RAW_SYNC());
> tmp_idx = ctx->idx * 4;
> /* 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..884eef1b3973 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -803,6 +803,15 @@ 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));
> + /*
> + * 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))
> + EMIT(PPC_RAW_SYNC());
> tmp_idx = ctx->idx * 4;
> /* load value from memory into TMP_REG_2 */
> if (size == BPF_DW)
> @@ -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.