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

[Jürgen Groß <jgross@xxxxxxxx>]

On 08.02.21 10:48, Jan Beulich wrote:
> 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@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);
> >                 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?
The problem solved here is the further processing using "p" multiple
times. p must not be silently replaced with u->ring_prod by the
compiler, so I probably should reword the commit message to say:

... in order to not allow the compiler to refetch p.


Juergen

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