RE: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling

[KY Srinivasan]

> -----Original Message-----
> From: Michael Kelley [mailto:mhkelley@xxxxxxxxxxx]
> Sent: Saturday, February 10, 2018 12:49 PM
> To: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx;
> vkuznets@xxxxxxxxxx; jasowang@xxxxxxxxxx;
> leann.ogasawara@xxxxxxxxxxxxx; marcelo.cerri@xxxxxxxxxxxxx; Stephen
> Hemminger <sthemmin@xxxxxxxxxxxxx>; KY Srinivasan
> <kys@xxxxxxxxxxxxx>
> Cc: Michael Kelley (EOSG) <Michael.H.Kelley@xxxxxxxxxxxxx>
> Subject: [PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling
> 
> Fix bugs in signaling the Hyper-V host when freeing space in the
> host->guest ring buffer:
> 
> 1. The interrupt_mask must not be used to determine whether to signal
>    on the host->guest ring buffer
> 2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
>    *after* pending_send_sz is read in order to avoid a race condition
> 3. Comparisons with pending_send_sz must treat the "equals" case as
>    not-enough-space
> 4. Don't signal if the pending_send_sz feature is not present. Older
>    versions of Hyper-V that don't implement this feature will poll.
> 
> Fixes: 03bad714a161 ("vmbus: more host signalling avoidance")
> Signed-off-by: Michael Kelley <mhkelley@xxxxxxxxxxx>
> ---
>  drivers/hv/ring_buffer.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 50e0714..b64be18 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -423,7 +423,11 @@ struct vmpacket_descriptor *
>  void hv_pkt_iter_close(struct vmbus_channel *channel)
>  {
>  	struct hv_ring_buffer_info *rbi = &channel->inbound;
> -	u32 orig_write_sz = hv_get_bytes_to_write(rbi);
> +	u32 curr_write_sz;
> +	u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ?
> +			(rbi->priv_read_index - rbi->ring_buffer-
> >read_index) :
> +			(rbi->ring_datasize - rbi->ring_buffer->read_index +
> +			rbi->priv_read_index);
> 
>  	/*
>  	 * Make sure all reads are done before we update the read index
> since
> @@ -446,27 +450,31 @@ void hv_pkt_iter_close(struct vmbus_channel
> *channel)
>  	 */
>  	virt_mb();
> 
> -	/* If host has disabled notifications then skip */
> -	if (rbi->ring_buffer->interrupt_mask)
> -		return;
> -
>  	if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) {
>  		u32 pending_sz = READ_ONCE(rbi->ring_buffer-
> >pending_send_sz);
> 
>  		/*
> +		 * Ensure the read of write_index in
> hv_get_bytes_to_write()
> +		 * happens after the read of pending_send_sz.
> +		 */
> +		virt_rmb();
We can avoid the read barrier by making the initialization of curr_write_sz conditional
on pending_send_sz being non-zero. Indeed you can make all the signaling code conditional
on pending_send_sz being non-zero.



> +		curr_write_sz = hv_get_bytes_to_write(rbi);
> +
> +		/*
>  		 * If there was space before we began iteration,
>  		 * then host was not blocked. Also handles case where
>  		 * pending_sz is zero then host has nothing pending
>  		 * and does not need to be signaled.
>  		 */
> -		if (orig_write_sz > pending_sz)
> +		if (curr_write_sz - delta > pending_sz)
>  			return;
> 
>  		/* If pending write will not fit, don't give false hope. */
> -		if (hv_get_bytes_to_write(rbi) < pending_sz)
> +		if (curr_write_sz <= pending_sz)
>  			return;
> +
> +		vmbus_setevent(channel);
>  	}
> 
> -	vmbus_setevent(channel);
>  }
>  EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
> --
> 1.8.3.1