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Message-ID: <fb71bcc5-77ad-698c-b025-36e1910f868f@linux.alibaba.com>
Date:   Thu, 10 Feb 2022 16:56:00 +0800
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Subject: Re: [PATCH net] net/smc: Avoid overwriting the copies of clcsock
 callback functions
To:     Tony Lu <tonylu@linux.alibaba.com>
Cc:     kgraul@linux.ibm.com, davem@davemloft.net, kuba@kernel.org,
        linux-s390@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1644415853-46641-1-git-send-email-guwen@linux.alibaba.com>
 <YgR9XrT8cATDP4Zx@TonyMac-Alibaba>
From:   Wen Gu <guwen@linux.alibaba.com>
In-Reply-To: <YgR9XrT8cATDP4Zx@TonyMac-Alibaba>
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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