linux-kernel - Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on

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Date:	Mon, 21 Mar 2016 09:19:01 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	devel@...uxdriverproject.org, olaf@...fle.de, apw@...onical.com,
	jasowang@...hat.com, stable@...r.kernel.org
Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read()

"K. Y. Srinivasan" <kys@...rosoft.com> writes:

> We need to issue a full memory barrier prior making a signalling
> decision.

Any reason this should be mb()? This is pretty strong and will probably
lead to performace regression ... and, btw, we have another mb() in
hv_ringbuffer_read().

Could you please describe the scenarion you're trying to protect against
so we could search for a better solution?

>
> 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();
>  	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
>  	/* If the other end is not blocked on write don't bother. */
>  	if (pending_sz == 0)

-- 
  Vitaly