linux-kernel - Re: [PATCH v2 2/4] ptp: vmclock: support device notifications

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Message-ID: <5315574130a7091d338fc2d7132fc4873f2186c6.camel@infradead.org>
Date: Tue, 02 Dec 2025 23:45:33 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: "Chalios, Babis" <bchalios@...zon.es>, "richardcochran@...il.com"
 <richardcochran@...il.com>, "andrew+netdev@...n.ch"
 <andrew+netdev@...n.ch>,  "davem@...emloft.net" <davem@...emloft.net>,
 "edumazet@...gle.com" <edumazet@...gle.com>,  "kuba@...nel.org"
 <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, 
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "Graf (AWS), Alexander" <graf@...zon.de>, "mzxreary@...inter.de"
	 <mzxreary@...inter.de>
Subject: Re: [PATCH v2 2/4] ptp: vmclock: support device notifications

On Tue, 2025-12-02 at 20:11 +0000, Chalios, Babis wrote:
> 
> @@ -402,8 +413,16 @@ static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,
>  
>  		/* Pairs with hypervisor wmb */
>  		virt_rmb();
> -		if (seq == le32_to_cpu(st->clk->seq_count))
> -			break;
> +		if (seq == le32_to_cpu(st->clk->seq_count)) {
> +			/*
> +			 * Either we updated fst->seq to seq (the latest version we observed)
> +			 * or someone else did (old_seq == seq), so we can break.
> +			 */
> +			if (atomic_try_cmpxchg(&fst->seq, &old_seq, seq) ||
> +			    old_seq == seq) {
> +				break;
> +			}
> +		}
>  
>  		if (ktime_after(ktime_get(), deadline))
>  			return -ETIMEDOUT;

I'm slightly confused that that compiles cleanly (as it does here).
Isn't the second argument to atomic_try_cmpxchg() an (int *), and isn't
old_seq a uint32_t? It's doing the right thing; I just didn't think it
would do the right thing *quietly*... ?

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