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@xxxxxxxxxx>
---
Changes in v1 -> v2:
v1: https://lore.kernel.org/all/20240507175439.119467-1-puranjay@xxxxxxxxxx/
- 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.
+ */
+ if (imm & BPF_FETCH && IS_ENABLED(CONFIG_SMP))
+ EMIT(PPC_RAW_SYNC());
/* 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))
+ EMIT(PPC_RAW_SYNC());
/* 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
