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** Branch linked: lp:ubuntu/trusty-proposed/linux-lts-wily

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https://bugs.launchpad.net/bugs/1556264

Title:
  [Hyper-V] vmbus: Fix a bug in hv_need_to_signal_on_read()

Status in linux package in Ubuntu:
  Fix Released
Status in linux-lts-vivid package in Ubuntu:
  In Progress
Status in linux source package in Trusty:
  Fix Committed
Status in linux-lts-vivid source package in Trusty:
  Fix Committed
Status in linux source package in Wily:
  Fix Committed
Status in linux source package in Xenial:
  Fix Released

Bug description:
  The following patch has been submitted upstream in response to
  investigation of customer issues with network connections hanging
  under high load.

  On the consumer side, we have interrupt driven flow management of the
  producer. It is sufficient to base the signalling decision on the
  amount of space that is available to write after the read is complete.
  The current code samples the previous available space and uses this in
  making the signalling decision. This state can be stale and is
  unnecessary. Since the state can be stale, we end up not signalling
  the host (when we should) and this can result in a hang. Fix this
  problem by removing the unnecessary check.

  I would like to thank Arseney Romanenko <arseneyr@xxxxxxxxxxxxx>
  for pointing out this bug.

  Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
  Tested-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
  Cc: <stable@xxxxxxxxxxxxxxx>
  ---
   drivers/hv/ring_buffer.c |    7 +++----
   1 files changed, 3 insertions(+), 4 deletions(-)

  diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
  index 5613e2b..085003a 100644
  --- a/drivers/hv/ring_buffer.c
  +++ b/drivers/hv/ring_buffer.c
  @@ -103,8 +103,7 @@ 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;
  @@ -120,7 +119,7 @@ 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 +454,7 @@ 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;
   }

  We have customers who are encountering this issue. Although this patch
  is not yet accepted upstream, we would like to get them a test kernel
  as soon as we can.

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