linux-kernel - Re: [PATCH 3/7] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on

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Message-ID: <20160401000017.GB10232@kroah.com>
Date:	Thu, 31 Mar 2016 17:00:17 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	olaf@...fle.de, apw@...onical.com, vkuznets@...hat.com,
	jasowang@...hat.com, stable@...r.kernel.org
Subject: Re: [PATCH 3/7] Drivers: hv: vmbus: Fix a bug in
 hv_need_to_signal_on_read()

On Wed, Mar 23, 2016 at 05:53:53PM -0700, K. Y. Srinivasan wrote:
> We need to issue a full memory barrier prior making a signalling decision.
> 
> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> Cc: stable@...r.kernel.org
> ---
>  drivers/hv/ring_buffer.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 2919395..67dc245 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -104,6 +104,7 @@ static bool hv_need_to_signal_on_read(struct hv_ring_buffer_info *rbi)
>  	u32 cur_write_sz;
>  	u32 pending_sz;
>  
> +	mb();

And, are you sure this is correct?  You better document the heck out of
this, why it's here, and what it is protecting.  "raw" mb() calls are
really rare for good reason.

thanks,

greg k-h