netdev - Re: [PATCH net 2/6] net/smc: set SOCK_NOSPACE when send_remaining but no sndbuf

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Message-ID: <62cd6d62-b233-4906-af4a-72127fc4c0f4@redhat.com>
Date: Tue, 3 Dec 2024 11:04:15 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Guangguan Wang <guangguan.wang@...ux.alibaba.com>, wenjia@...ux.ibm.com,
 jaka@...ux.ibm.com, alibuda@...ux.alibaba.com, tonylu@...ux.alibaba.com,
 guwen@...ux.alibaba.com, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, horms@...nel.org
Cc: linux-rdma@...r.kernel.org, linux-s390@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/6] net/smc: set SOCK_NOSPACE when send_remaining but
 no sndbuf_space left



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