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++.
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