> -----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. Konstantin