linux-kernel - Re: [PATCH net] net/smc: Avoid overwriting the copies of clcsock callback functions

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Message-ID: <fb71bcc5-77ad-698c-b025-36e1910f868f@linux.alibaba.com>
Date:   Thu, 10 Feb 2022 16:56:00 +0800
From:   Wen Gu <guwen@...ux.alibaba.com>
To:     Tony Lu <tonylu@...ux.alibaba.com>
Cc:     kgraul@...ux.ibm.com, davem@...emloft.net, kuba@...nel.org,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net/smc: Avoid overwriting the copies of clcsock
 callback functions



On 2022/2/10 10:50 am, Tony Lu wrote:

> I am wondering that there is a potential racing. If ->use_fallback is
> setted to true, but the rest of replacing process is on the way, others
> who tested and passed ->use_fallback, they would get old value before
> replacing.
> 

Thanks for your comments.

I understand your concern. But when I went through all the places that
check for smc->use_fallback, I haven't found the exact potential racing
point. Please point out if I missed something. Thank you.

In my humble opinion, most of the operations after smc->use_fallback check
have no direct relationship with what did in smc_switch_to_fallback() (the
replacement of clcsock callback functions), except for which in smc_sendmsg(),
smc_recvmsg() and smc_sendpage():

smc_sendmsg():

	if (smc->use_fallback) {
		rc = smc->clcsock->ops->sendmsg(smc->clcsock, msg, len);
	}

smc_recvmsg():

	if (smc->use_fallback) {
		rc = smc->clcsock->ops->recvmsg(smc->clcsock, msg, len, flags);
	}

smc_sendpage():

	if (smc->use_fallback) {
		rc = kernel_sendpage(smc->clcsock, page, offset,
				     size, flags);
	}

If smc->use_fallback is set to true, but callback functions (sk_data_ready ...)
of clcsock haven't been replaced yet at this moment, there may be a racing as
you described.

But it won't happen, because fallback must already be done before sending and receiving.

What do you think about it?

Thanks,
Wen Gu