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

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

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++.

Ross