Re: [dpdk-dev] [PATCH v2] rte_ethdev: safer memory access by calling Rx callback

["tgw_team(腾讯网关团队)" <tgw_team@tencent.com>]

Thank you raising your concerns.

I mean, the original wrong code, but using -O3 optimization yielded a correct result.
My patch makes the effects of -O3 and -O0 consistent.

Unlike other signals that require busy wait, this callback pointer only needs to be read once. So I don't think memory barriers and volatile are needed here.
    
>On Thu,  5 Mar 2020 01:33:49 +0800
>ZY Qiu <quzeyao@gmail.com> wrote:
>
>> When compiling with -O0,
>> the compiler does not optimize two memory accesses into one.
>> Leads to accessing a null pointer when queue post Rx burst callback
>> removal while traffic is running.
>> See rte_eth_tx_burst function.
>> 
>> Signed-off-by: ZY Qiu <tgw_team@tencent.com>
>
>This is a problem many places in DPDK. You said it was related to -O0
>but that is just what is causing a more generic problem to be exposed.
>Your solution is not sufficient.
>
>DPDK is sloppy in several places in handling memory ordering issues     
>    https://en.wikipedia.org/wiki/Memory_ordering
>
>It should have a macro to do RTE_READ_ONCE(). Several drives have this,
>the Linux kernel has it, Liburcu has it.
>
>The macro RTE_READ_ONCE() can then be used in many places in DPDK.