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

[Michael Ellerman]

Puranjay Mohan <puranjay@xxxxxxxxxx> writes:
> Naveen N Rao <naveen@xxxxxxxxxx> writes:
>> On Mon, May 13, 2024 at 10:02:48AM GMT, Puranjay Mohan wrote:
>>> 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")
>>
>> As I noted in v2, I think that is the wrong commit. This fixes the below 
>
> Sorry for missing this. Would this need another version or your message
> below will make it work with the stable process?

No need for another version. b4 should pick up those tags, or if not
I'll add them by hand.

cheers