linux-kernel - Re: [PATCH 7/7] xen/evtchn: read producer index only once

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Date:   Mon, 8 Feb 2021 10:48:48 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once

On 06.02.2021 11:49, Juergen Gross wrote:
> In evtchn_read() use READ_ONCE() for reading the producer index in
> order to avoid the compiler generating multiple accesses.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  drivers/xen/evtchn.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> index 421382c73d88..f6b199b597bf 100644
> --- a/drivers/xen/evtchn.c
> +++ b/drivers/xen/evtchn.c
> @@ -211,7 +211,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
>  			goto unlock_out;
>  
>  		c = u->ring_cons;
> -		p = u->ring_prod;
> +		p = READ_ONCE(u->ring_prod);
>  		if (c != p)
>  			break;

Why only here and not also in

		rc = wait_event_interruptible(u->evtchn_wait,
					      u->ring_cons != u->ring_prod);

or in evtchn_poll()? I understand it's not needed when
ring_prod_lock is held, but that's not the case in the two
afaics named places. Plus isn't the same then true for
ring_cons and ring_cons_mutex, i.e. aren't the two named
places plus evtchn_interrupt() also in need of READ_ONCE()
for ring_cons?

Jan