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

[Jürgen Groß <jgross@xxxxxxxx>]

On 08.02.21 12:40, Julien Grall wrote:
> 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@xxxxxxxx>
> > ---
> >   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()?
Only in case I'd consider the compiler needing multiple memory
accesses for doing the update (see my reply to Jan's comment on this
patch).


Juergen

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