On 2024/12/3 18:04, Paolo Abeni wrote: > > > On 11/28/24 13:14, Guangguan Wang wrote: >> When application sending data more than sndbuf_space, there have chances >> application will sleep in epoll_wait, and will never be wakeup again. This >> is caused by a race between smc_poll and smc_cdc_tx_handler. >> >> application tasklet >> smc_tx_sendmsg(len > sndbuf_space) | >> epoll_wait for EPOLL_OUT,timeout=0 | >> smc_poll | >> if (!smc->conn.sndbuf_space) | >> | smc_cdc_tx_handler >> | atomic_add sndbuf_space >> | smc_tx_sndbuf_nonfull >> | if (!test_bit SOCK_NOSPACE) >> | do not sk_write_space; >> set_bit SOCK_NOSPACE; | >> return mask=0; | >> >> Application will sleep in epoll_wait as smc_poll returns 0. And >> smc_cdc_tx_handler will not call sk_write_space because the SOCK_NOSPACE >> has not be set. If there is no inflight cdc msg, sk_write_space will not be >> called any more, and application will sleep in epoll_wait forever. >> So set SOCK_NOSPACE when send_remaining but no sndbuf_space left in >> smc_tx_sendmsg, to ensure call sk_write_space in smc_cdc_tx_handler >> even when the above race happens. > > I think it should be preferable to address the mentioned race the same > way as tcp_poll(). i.e. checking again smc->conn.sndbuf_space after > setting the NOSPACE bit with appropriate barrier, see: > > https://elixir.bootlin.com/linux/v6.12.1/source/net/ipv4/tcp.c#L590 > > that will avoid additional, possibly unneeded atomic operation in the tx > path (the application could do the next sendmsg()/poll() call after that > the send buf has been freed) and will avoid some code duplication. > > Cheers, > > Paolo Hi, Paolo Thanks for advice, and the way in tcp_poll() seems a better solution for this race. I will retest it, and resend a new version of patch if it works. Thanks, Guangguan Wang
