linux-kernel - RE: [PATCH 1/1] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on

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Date:	Sat, 2 Apr 2016 20:58:52 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	KY Srinivasan <kys@...rosoft.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
CC:	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH 1/1] Drivers: hv: vmbus: Fix a bug in
 hv_need_to_signal_on_read()



> -----Original Message-----
> From: K. Y. Srinivasan [mailto:kys@...rosoft.com]
> Sent: Friday, March 11, 2016 12:39 PM
> To: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> vkuznets@...hat.com; jasowang@...hat.com
> Cc: KY Srinivasan <kys@...rosoft.com>; stable@...r.kernel.org
> Subject: [PATCH 1/1] Drivers: hv: vmbus: Fix a bug 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 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@...rosoft.com>
> for pointing out this bug.

Greg,

Please drop this patch. I am going to update and send the patch.

K. Y
> 
> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> Tested-by: Dexuan Cui <decui@...rosoft.com>
> Cc: <stable@...r.kernel.org>
> ---
>  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;
>  }
> --
> 1.7.4.1