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Date:   Wed, 20 Mar 2019 13:06:19 -0700
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     Kimberly Brown <kimbrownkd@gmail.com>
Cc:     Michael Kelley <mikelley@microsoft.com>,
        Long Li <longli@microsoft.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] Drivers: hv: vmbus: Fix race condition with new
 ring_buffer_info mutex
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On Sat, 16 Mar 2019 21:49:28 -0400
Kimberly Brown <kimbrownkd@gmail.com> wrote:

> On Thu, Mar 14, 2019 at 03:45:33PM -0700, Stephen Hemminger wrote:
> > On Thu, 14 Mar 2019 13:05:15 -0700
> > "Kimberly Brown" <kimbrownkd@gmail.com> wrote:
> >   
> > > Fix a race condition that can result in a ring buffer pointer being set
> > > to null while a "_show" function is reading the ring buffer's data. This
> > > problem was discussed here:
> > > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org
> > > %2Flkml%2F2018%2F10%2F18%2F779&amp;data=02%7C01%7Csthemmin%40microsoft.com
> > > %7C1d7557d667b741bdbb6008d6a8b8620f%7C72f988bf86f141af91ab2d7cd011db47%7C1
> > > %7C0%7C636881907217609564&amp;sdata=1bUbLaxsODANM7lCBR8lxyYajNpufuwUW%2FOl
> > > vtGu2hU%3D&amp;reserved=0
> > > 
> > > To fix the race condition, add a new mutex lock to the
> > > "hv_ring_buffer_info" struct. Add a new function,
> > > "hv_ringbuffer_pre_init()", where a channel's inbound and outbound
> > > ring_buffer_info mutex locks are initialized.
> > >
> > >  ... snip ...    
> > 
> > Adding more locks will solve the problem but it seems like overkill.
> > Why not either use a reference count or an RCU style access for the
> > ring buffer?  
> 
> I agree that a reference count or RCU could also solve this problem.
> Using mutex locks seemed like the most straightforward solution, but
> I'll certainly switch to a different approach if it's better!
> 
> Are you concerned about the extra memory required for the mutex locks,
> read performance, or something else?

Locks in control path are ok, but my concern is performance of the
data path which puts packets in/out of rings. To keep reasonable performance,
no additional locking should be added in those paths.

So if data path is using RCU, can/should the control operations also
use it?