[Xen-devel] [[PATCH -v2]] XenBus: Don't wait for the producer to fill the ring if

[Anshul Makkar <anshul.makkar@xxxxxxxxxx>]

The condition: if there is a space in the ring then wait for the producer
to fill the ring also evaluates to true even if the ring if full. It
leads to a deadlock where producer is waiting for consumer
to consume the items and consumer is waiting for producer to fill the ring.

Fix for the issue: check if the ring is full and then break from
the loop to consume the items from the ring.
eg. case: prod = 1272, cons = 248.

Signed-off-by: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
---
v2:
 * resolved the coding style issue.
 * modified the "if" condition statement to make it simpler.

 tools/firmware/hvmloader/xenbus.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/firmware/hvmloader/xenbus.c 
b/tools/firmware/hvmloader/xenbus.c
index 448157d..2b89a56 100644
--- a/tools/firmware/hvmloader/xenbus.c
+++ b/tools/firmware/hvmloader/xenbus.c
@@ -141,7 +141,19 @@ static void ring_read(char *data, uint32_t len)
         /* Don't overrun the producer pointer */
         while ( (part = MASK_XENSTORE_IDX(rings->rsp_prod -
                                           rings->rsp_cons)) == 0 )
+        {
+            /*
+             * Don't wait for producer to fill the ring if it is already full.
+             * Condition happens when you write string > 1K into the ring.
+             * eg case prod=1272 cons=248.
+             */
+            if ( rings->rsp_prod - rings->rsp_cons == XENSTORE_RING_SIZE )
+            {
+                part = XENSTORE_RING_SIZE;
+                break;
+            }
             ring_wait();
+        }
         /* Don't overrun the end of the ring */
         if ( part > (XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(rings->rsp_cons)) )
             part = XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(rings->rsp_cons);
-- 
2.7.4


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