1 files changed, 20 insertions, 6 deletions

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 5613e2b5cff7..a40a73a7b71d 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -103,15 +103,29 @@ static bool hv_need_to_signal(u32 old_write, struct hv_ring_buffer_info *rbi)
  *    there is room for the producer to send the pending packet.
  */
 
-static bool hv_need_to_signal_on_read(u32 prev_write_sz,
-                                      struct hv_ring_buffer_info *rbi)
+static bool hv_need_to_signal_on_read(struct hv_ring_buffer_info *rbi)
 {
         u32 cur_write_sz;
         u32 r_size;
-        u32 write_loc = rbi->ring_buffer->write_index;
+        u32 write_loc;
         u32 read_loc = rbi->ring_buffer->read_index;
-        u32 pending_sz = rbi->ring_buffer->pending_send_sz;
+        u32 pending_sz;
 
+        /*
+         * Issue a full memory barrier before making the signaling decision.
+         * Here is the reason for having this barrier:
+         * If the reading of the pend_sz (in this function)
+         * were to be reordered and read before we commit the new read
+         * index (in the calling function)  we could
+         * have a problem. If the host were to set the pending_sz after we
+         * have sampled pending_sz and go to sleep before we commit the
+         * read index, we could miss sending the interrupt. Issue a full
+         * memory barrier to address this.
+         */
+        mb();
+
+        pending_sz = rbi->ring_buffer->pending_send_sz;
+        write_loc = rbi->ring_buffer->write_index;
         /* If the other end is not blocked on write don't bother. */
         if (pending_sz == 0)
                 return false;
@@ -120,7 +134,7 @@ static bool hv_need_to_signal_on_read(u32 prev_write_sz,
         cur_write_sz = write_loc >= read_loc ? r_size - (write_loc - read_loc) :
                         read_loc - write_loc;
 
-        if ((prev_write_sz < pending_sz) && (cur_write_sz >= pending_sz))
+        if (cur_write_sz >= pending_sz)
                 return true;
 
         return false;
@@ -455,7 +469,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
         /* Update the read index */
         hv_set_next_read_location(inring_info, next_read_location);
 
-        *signal = hv_need_to_signal_on_read(bytes_avail_towrite, inring_info);
+        *signal = hv_need_to_signal_on_read(inring_info);
 
         return ret;
 }