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

Jan Beulich
2021-02-08T10:00:59Z

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@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);
> 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