On Wed, Jun 15, 2016 at 04:07:20PM +0200, Ivan Boule wrote: > On 06/15/2016 03:29 PM, Bruce Richardson wrote: > >On Wed, Jun 15, 2016 at 12:40:16PM +0000, Ananyev, Konstantin wrote: > >>Hi Ivan, > >> > >>>-----Original Message----- > >>>From: Ivan Boule [mailto:ivan.boule at 6wind.com] > >>>Sent: Wednesday, June 15, 2016 1:15 PM > >>>To: Thomas Monjalon; Ananyev, Konstantin > >>>Cc: Pattan, Reshma; dev at dpdk.org > >>>Subject: Re: [dpdk-dev] [PATCH v9 1/8] ethdev: use locks to protect Rx/Tx > >>>callback lists > >>> > >>>On 06/15/2016 10:48 AM, Thomas Monjalon wrote: > >>> > >>>>> > >>>>>>I think the read access would need locking but we do not want it > >>>>>>in fast path. > >>>>> > >>>>>I don't think it would be needed. > >>>>>As I said - read/write interaction didn't change from what we have right > >>>>>now. > >>>>>But if you have some particular scenario in mind that you believe would > >>>>>cause > >>>>>a race condition - please speak up. > >>>> > >>>>If we add/remove a callback during a burst? Is it possible that the next > >>>>pointer would have a wrong value leading to a crash? > >>>>Maybe we need a comment to state that we should not alter burst > >>>>callbacks while running burst functions. > >>>> > >>> > >>>Hi Reshma, > >>> > >>>You claim that the "rte_eth_rx_cb_lock" does not need to be hold in the > >>>function "rte_eth_rx_burst()" while parsing the "post_rx_burst_cbs" list > >>>of RX callbacks associated with the polled RX queue to safely remove RX > >>>callback(s) in parallel. > >>>The problem is not [only] with the setting and the loading of "cb->next" > >>>that you assume to be atomic operations, which is certainly true on most > >>>CPUs. > >>>I see the 2 important following issues: > >>> > >>>1) the "rte_eth_rxtx_callback" data structure associated with a removed > >>>RX callback could still be accessed in the callback parsing loop of the > >>>function "rte_eth_rx_burst()" after having been freed in parallel. > >>> > >>>BUT SUCH A BAD SITUATION WILL NOT CURRENTLY HAPPEN, THANKS TO THE NICE > >>>MEMORY LEAK BUG in the function "rte_eth_remove_rx_callback()" that > >>>does not free the "rte_eth_rxtx_callback" data structure associated with > >>>the removed callback ! > >> > >>Yes, though it is documented behaviour, someone can probably > >>refer it as a feature, not a bug ;) > >> > > > >+1 > >This is definitely not a bug, this is absolutely by design. One may argue > >with > >the design, but it was done for a definite reason, so as to avoid paying the > >penalty of having locks. It pushes more responsibility onto the app, but it > >does allow the app to choose the best solution for managing the freeing of > >memory for its situation. The alternative is to force all apps to pay the > >cost > >of having locks, even if better options for freeing the memory are available. > > > >/Bruce > > > > -1 (not to say 0xFFFFFFFF) > > This is definitely an API design bug ! > I would say that if you don't know how to free a resource that you allocate, > it is very likely that you are wrong allocating it. > And this is exactly what happens here with RX/TX callback data structures. > This problem can easily be addressed by just changing the API as follows: > > Change > void * > rte_eth_add_rx_callback(uint8_t port_id, uint16_t queue_id, > rte_rx_callback_fn fn, void *user_param) > > to > int > rte_eth_add_rx_callback(uint8_t port_id, uint16_t queue_id, > struct rte_eth_rxtx_callback *cb) > > In addition of solving the problem, this approach makes the API consistent > and let the application allocate "rte_eth_rxtx_callback" data structures > through any appropriate mean. >
That looks like a reasonable change to me. It keeps the important part of the existing API behaviour, while making the API more consistent. /Bruce
