linux-kernel - Re: [PATCH v3 3/3] Drivers: hv: vmbus: Fix race condition with new ring_buffer

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

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@...il.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?

Thanks,
Kim