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Re: [PATCH 02/12] evtchn: avoid race in get_xen_consumer()



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 <jbeulich@xxxxxxxx>
---
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



 


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