Merge tag 'char-misc-4.6-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh...

[deliverable/linux.git] / drivers / hv / ring_buffer.c

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 5613e2b5cff7759861a1e2d7ad2748f7626d16da..a40a73a7b71da359574bf4061d80340b1caa30a4 100644 (file)
--- 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;
 }