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 <jgr...@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
