Re: [PATCH] drivers: net: xgene: fix: Out of order descriptor bytes read

[Iyappan Subramanian <isubramanian@apm.com>]

On Thu, Jan 22, 2015 at 2:50 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Thu, 2015-01-22 at 12:03 -0800, Iyappan Subramanian wrote:
>> This patch fixes the following kernel crash,
>>
>>       WARNING: CPU: 2 PID: 0 at net/ipv4/tcp_input.c:3079 tcp_clean_rtx_queue+0x658/0x80c()
>>       Call trace:
>
>>
>> Software writes poison data into the descriptor bytes[15:8] and upon
>> receiving the interrupt, if those bytes are overwritten by the hardware with
>> the valid data, software also reads bytes[7:0] and executes receive/tx
>> completion logic.
>>
>> If the CPU executes the above two reads in out of order fashion, then the
>> bytes[7:0] will have older data and causing the kernel panic.  We have to
>> force the order of the reads and thus this patch introduces read memory
>> barrier between these reads.
>>
>> Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
>> Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
>> Tested-by: Mark Langsdorf <mlangsdo@redhat.com>
>> ---
>>  drivers/net/ethernet/apm/xgene/xgene_enet_main.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
>> index 83a5028..3622cdb 100644
>> --- a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
>> +++ b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
>> @@ -369,6 +369,8 @@ static int xgene_enet_process_ring(struct xgene_enet_desc_ring *ring,
>>               if (unlikely(xgene_enet_is_desc_slot_empty(raw_desc)))
>>                       break;
>>
>> +             /* read fpqnum field after dataaddr field */
>> +             smp_rmb();
>>               if (is_rx_desc(raw_desc))
>>                       ret = xgene_enet_rx_frame(ring, raw_desc);
>>               else
>
> Reading your changelog, it looks like you need a plain rmb() here.

rmb() translates into dsb, which in arm64 serializes everything
including instructions and thus expensive compared to dmb.

Do you see any issue with smp_rmb() (which translates into dmb) ?

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