[PATCH bpf-next] s390/bpf: Fully order atomic "add", "and", "or" and "xor"

[Ilya Leoshkevich <iii@linux.ibm.com>]

BPF_ATOMIC_OP() macro documentation states that "BPF_ADD | BPF_FETCH"
should be the same as atomic_fetch_add(), which is currently not the
case on s390x: the synchronization instruction "bcr 14,0" is missing.

This should not be a problem in practice, because s390x is allowed to
reorder only stores with subsequent fetches from different addresses.
Still, just to be on the safe side, and also for consistency, emit the
synchronization instruction.

Note that it's not required to do this for BPF_XCHG and BPF_CMPXCHG,
because COMPARE AND SWAP performs serialization itself.

Fixes: ba3b86b9cef0 ("s390/bpf: Implement new atomic ops")
Reported-by: Puranjay Mohan <puranjay12@gmail.com>
Closes: https://lore.kernel.org/bpf/mb61p34qvq3wf.fsf@kernel.org/
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index fa2f824e3b06..a0dfb3f665ab 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1427,6 +1427,8 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 	EMIT6_DISP_LH(0xeb000000, is32 ? (op32) : (op64),		\
 		      (insn->imm & BPF_FETCH) ? src_reg : REG_W0,	\
 		      src_reg, dst_reg, off);				\
+	/* bcr 14,0 - see atomic_fetch_{add,and,or,xor}() */		\
+	_EMIT2(0x07e0);							\
 	if (is32 && (insn->imm & BPF_FETCH))				\
 		EMIT_ZERO(src_reg);					\
 } while (0)
-- 
2.45.0