2016-06-15 09:54, Ananyev, Konstantin: > > > -----Original Message----- > > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > Sent: Wednesday, June 15, 2016 9:49 AM > > To: 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 > > > > 2016-06-15 08:37, Ananyev, Konstantin: > > > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > > > 2016-06-15 05:30, Pattan, Reshma: > > > > > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > > > > > 2016-06-14 10:38, Reshma Pattan: > > > > > > > Added spinlocks around add/remove logic of Rx and Tx callbacks to > > > > > > > avoid corruption of callback lists in multithreaded context. > > > > > > > > > > > > > > Signed-off-by: Reshma Pattan <reshma.pattan at intel.com> > > > > > > > > > > > > Why cb->next is not locked in burst functions? > > > > > It is safe to do "read access" here and doesn't require any locking > > > > > as rx/tx burst is initiated by only local user(control plane) > > thread. > > > > > > > > > > > Just protecting add/remove but not its usage seems useless. > > > > > Here locks were required around add/remove to protect "write access" > > > > > because write to callback list is now done from 2 > > threads > > > > > i.e. one from local user thread(control plane) and another from pdump > > > > > control thread(initiated by remote pdump request). > > > > > > > > So read and write can be done by different threads. > > > > > > Yes, and this is possible even in current DPDK version (16.04). > > > What is added by Reshma's patch - now it is possible to have concurrent > > > write > > > from 2 different thread to that list. > > > > > > > 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. > > Current status (16.04): > It is safe to add/remove RX/TX callbacks while > another thread is doing simultaneously RX/TX burst over same queue. > I.E: it is supposed to be safe to invoke > rte_eth_add(/remove)_rx(/tx)_callback() and > rte_eth_rx_burst()/rte_eth_tx_burst() > from different threads simultaneously. > Though it is not safe to free/modify that rte_eth_rxtx_callback while current > rte_eth_rx_burst()/rte_eth_tx_burst() are still active. > That exactly what comments for rte_eth_remove_rx_callback() say: > > * Note: the callback is removed from the callback list but it isn't freed > * since the it may still be in use. The memory for the callback can be > * subsequently freed back by the application by calling rte_free(): > * > * - Immediately - if the port is stopped, or the user knows that no > * callbacks are in flight e.g. if called from the thread doing RX/TX > * on that queue. > * > * - After a short delay - where the delay is sufficient to allow any > * in-flight callbacks to complete. > > In other words, right now there only way to know for sure that it is safe > to free the removed callback - is to stop the port. > > Does it need to be changed, so when rte_eth_remove_rx_callback() returns > user can safely free the callback (or even better rte_eth_remove_rx_callback > free the callback for us)? > In my opinion - yes. > Though, I think, it has nothing to do with pdump patches, and I think should > be a matter > for separate a patch/discussion. > > Now with pdump library introduction - there is possibility that 2 different > threads > can try to add/remove callbacks for the same queue simultaneously. > First one - thread executing control requests from local user, > second one - pdump control thread executing pdump requests from pdump client. > That lock is introduced to avoid race condition between such 2 threads: > i.e. to prevent multiple threads to modify same list simultaneously. > It is not intended to synchronise read/write accesses to the list, see above.
OK thanks for the explanations
