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

[Ross Lagerwall <ross.lagerwall@citrix.com>]

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@suse.com>
> ---
> 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