[ViewVC] Diff of: cvs/libev/ev

Comparing libev/ev_linuxaio.c (file contents):
Revision 1.10 by root, Sun Jun 23 02:02:24 2019 UTC vs.
Revision 1.13 by root, Sun Jun 23 23:38:38 2019 UTC

 static int
 linuxaio_get_events_from_ring (EV_P)
 {
   struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx;
-  unsigned head = ring->head;
+  /* the kernel reads and writes both of these variables, */
+  /* as a C extension, we assume that volatile use here */
+  /* both makes reads atomic and once-only */
+  unsigned head = *(volatile unsigned *)&ring->head;
   unsigned tail = *(volatile unsigned *)&ring->tail;
   if (head == tail)
     return 0;
   if (ecb_expect_false (ring->magic != AIO_RING_MAGIC)
                         || ring->incompat_features != AIO_RING_INCOMPAT_FEATURES
                         || ring->header_length != sizeof (struct aio_ring)) /* TODO: or use it to find io_event[0]? */
     return 0;
+  /* make sure the events up to tail are visible */
   ECB_MEMORY_FENCE_ACQUIRE;
   /* parse all available events, but only once, to avoid starvation */
   if (tail > head) /* normal case around */
     linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head);
     {
       linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head);
       linuxaio_parse_events (EV_A_ ring->io_events, tail);
     }
-  ring->head = tail;
+  /* as an extension to C, we hope that the volatile will makethis atomic and once-only */
+  *(volatile unsigned *)&ring->head = tail;
+  /* make sure kernel can see our new head value - probably not required */
+  ECB_MEMORY_FENCE_RELEASE;
   return 1;
 }
 /* read at least one event from kernel, or timeout */

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