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Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
To:     Juergen Gross <jgross@suse.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-8-jgross@suse.com>
From:   Julien Grall <julien@xen.org>
Message-ID: <8032d8a9-b28f-95e1-a5a8-e955ada4dc0a@xen.org>
Date:   Mon, 8 Feb 2021 11:40:24 +0000
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On 06/02/2021 10:49, 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@suse.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);
For consistency, don't you also need the write side in 
evtchn_interrupt() to use WRITE_ONCE()?

>   		if (c != p)
>   			break;
>   
> 

-- 
Julien Grall