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

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Message-ID: <87zitq7s5v.fsf@vitty.brq.redhat.com>
Date:	Tue, 22 Mar 2016 10:55:56 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	KY Srinivasan <kys@...rosoft.com>
Cc:	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf\@aepfle.de" <olaf@...fle.de>,
	"apw\@canonical.com" <apw@...onical.com>,
	"jasowang\@redhat.com" <jasowang@...hat.com>,
	"stable\@vger.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read()

KY Srinivasan <kys@...rosoft.com> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
>> Sent: Monday, March 21, 2016 1:19 AM
>> To: KY 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?
>
> If the reading of the pend_sz (in the function hv_need_to_signal_on_read)
> were to be reordered and read before we commit the new read index we could 
> have a problem.


If these are two reads we can add a lightweight barrier just preventing
compiler from reordering (e.g. smp_rmb()), right?

> 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.

so write and then we read and we need to prevent reordering... not sure
how to get rid on mb() then ...

-- 
  Vitaly