Re: [PATCH net 2/6] net/smc: set SOCK_NOSPACE when send_remaining but no sndbuf_space left

[Paolo Abeni <pabeni@redhat.com>]

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