> -----Original Message-----
> From: K. Y. Srinivasan [mailto:k...@microsoft.com]
> Sent: Saturday, April 2, 2016 3:44 PM
> To: gre...@linuxfoundation.org; linux-ker...@vger.kernel.org;
> de...@linuxdriverproject.org; o...@aepfle.de; a...@canonical.com;
> vkuzn...@redhat.com; jasow...@redhat.com
> Cc: KY Srinivasan <k...@microsoft.com>; sta...@vger.kernel.org
> Subject: [PATCH 1/1] Drivers: hv: vmbus: Fix signaling logic in
> hv_need_to_signal_on_read()
>
> On the consumer side, we have interrupt driven flow management of the
> producer. It is sufficient to base the signaling 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 signaling decision. This state can be stale and is
> unnecessary. Since the state can be stale, we end up not signaling
> 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 <arsen...@microsoft.com> for pointing out this issue.
>
> Also, issue a full memory barrier before making the signaling descision
> to correctly deal with potential reordering of the write (read index)
> followed by the read of pending_sz.
Greg,
Please drop this; I sent the wrong version of the patch. Sorry for
The confusion.
K. Y
>
> Signed-off-by: K. Y. Srinivasan <k...@microsoft.com>
> Tested-by: Dexuan Cui <de...@microsoft.com>
> Cc: <sta...@vger.kernel.org>
> ---
> drivers/hv/ring_buffer.c | 20 ++++++++++++++++----
> 1 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 5613e2b..e00b632 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;
> @@ -112,6 +111,19 @@ static bool hv_need_to_signal_on_read(u32
> prev_write_sz,
> u32 read_loc = rbi->ring_buffer->read_index;
> u32 pending_sz = rbi->ring_buffer->pending_send_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();
> +
> /* If the other end is not blocked on write don't bother. */
> if (pending_sz == 0)
> return false;
> @@ -120,7 +132,7 @@ 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 +467,7 @@ 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;
> }
> --
> 1.7.4.1
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