Re: [PATCH bpf-next v2] s390/bpf: Emit a barrier for BPF_FETCH instructions

[Puranjay Mohan <puranjay12@gmail.com>]

Ilya Leoshkevich <iii@linux.ibm.com> writes:

> 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 serialization instruction "bcr 14,0" is missing.
> This applies to "and", "or" and "xor" variants too.
>
> s390x is allowed to reorder stores with subsequent fetches from
> different addresses, so code relying on BPF_FETCH acting as a barrier,
> for example:
>
>   stw [%r0], 1
>   afadd [%r1], %r2
>   ldxw %r3, [%r4]
>
> may be broken. Fix it by emitting "bcr 14,0".
>
> Note that a separate serialization instruction is not needed 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>

Reviewed-by: Puranjay Mohan <puranjay@kernel.org>

Thanks,
Puranjay