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

Julien Grall
2021-02-08T12:03:34Z

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