Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices

[Jürgen Groß <jgross@xxxxxxxx>]

On 17.02.21 14:29, Ross Lagerwall wrote:
> On 2021-02-11 10:16, Juergen Gross wrote:
> > For avoiding read- and write-tearing by the compiler use READ_ONCE()
> > and WRITE_ONCE() for accessing the ring indices in evtchn.c.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > V2:
> > - modify all accesses (Julien Grall)
> > ---
> >   drivers/xen/evtchn.c | 25 ++++++++++++++++---------
> >   1 file changed, 16 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> > index 421382c73d88..620008f89dbe 100644
> > --- a/drivers/xen/evtchn.c
> > +++ b/drivers/xen/evtchn.c
> > @@ -162,6 +162,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
> >   {
> >         struct user_evtchn *evtchn = data;
> >         struct per_user_data *u = evtchn->user;
> > +       unsigned int prod, cons;
> >   
> >   	WARN(!evtchn->enabled,
> >              "Interrupt for port %u, but apparently not enabled; per-user %p\n",
> > @@ -171,10 +172,14 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
> >   
> >   	spin_lock(&u->ring_prod_lock);
> >   
> > -	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
> > -               *evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
> > +       prod = READ_ONCE(u->ring_prod);
> > +       cons = READ_ONCE(u->ring_cons);
> > +
> > +       if ((prod - cons) < u->ring_size) {
> > +               *evtchn_ring_entry(u, prod) = evtchn->port;
> >                 smp_wmb(); /* Ensure ring contents visible */
> > -               if (u->ring_cons == u->ring_prod++) {
> > +               if (cons == prod++) {
> > +                       WRITE_ONCE(u->ring_prod, prod);
> >                         wake_up_interruptible(&u->evtchn_wait);
> >                         kill_fasync(&u->evtchn_async_queue,
> >                                     SIGIO, POLL_IN);
> This doesn't work correctly since now u->ring_prod is only updated if cons == 
> prod++.
Right. Thanks for noticing.


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: application/pgp-keys

Attachment: OpenPGP_signature
Description: OpenPGP digital signature