Hi Jan,
On 28/09/2020 11:56, Jan Beulich wrote:
There's no global lock around the updating of this global piece of data.
Make use of cmpxchg() to avoid two entities racing with their updates.
Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
TBD: Initially I used cmpxchgptr() here, until realizing Arm doesn't
have it. It's slightly more type-safe than cmpxchg() (requiring
all arguments to actually be pointers), so I now wonder whether Arm
should gain it (perhaps simply by moving the x86 implementation to
xen/lib.h), or whether we should purge it from x86 as being
pointless.
I would be happy with an implementation of cmpxchgptr() in xen/lib.h.
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -57,7 +57,8 @@
* with a pointer, we stash them dynamically in a small lookup array which
* can be indexed by a small integer.
*/
-static xen_event_channel_notification_t xen_consumers[NR_XEN_CONSUMERS];
+static xen_event_channel_notification_t __read_mostly
+ xen_consumers[NR_XEN_CONSUMERS];
This doesn't seem directly related to the changes below. Can you explain
it in the commit message?
/* Default notification action: wake up from wait_on_xen_event_channel(). */
static void default_xen_notification_fn(struct vcpu *v, unsigned int port)
@@ -81,7 +82,7 @@ static uint8_t get_xen_consumer(xen_even
for ( i = 0; i < ARRAY_SIZE(xen_consumers); i++ )
{
if ( xen_consumers[i] == NULL )
- xen_consumers[i] = fn;
+ (void)cmpxchg(&xen_consumers[i], NULL, fn);
This wants an explanation in the code. Maybe:
"As there is no global lock, the cmpxchg() will prevent race between two
callers."
if ( xen_consumers[i] == fn )
break;
}
Cheers,
--
Julien Grall
