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Date:   Thu, 28 Mar 2019 11:42:52 -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-hyperv@vger.kernel.org>,
        "linux-kernel@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 Thu, 28 Mar 2019 00:30:57 -0400
Kimberly Brown <kimbrownkd@gmail.com> wrote:

> On Thu, Mar 21, 2019 at 04:04:20PM +0000, Michael Kelley wrote:
> > From: Kimberly Brown <kimbrownkd@gmail.com>  Sent: Wednesday, March 20, 2019 8:48 PM  
> > > > > > 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?  
> > >   
> 
> 
> Hi Stephen,
> 
> Do you have any additional questions or suggestions for this race
> condition and the mutex locks? I think that your initial questions were
> addressed in the responses below. If there's anything else, please let
> me know!
> 
> Thanks,
> Kim
> 
> 
> > > The data path doesn't use RCU to protect the ring buffers.  
> > 
> > My $.02:  The mutex is obtained only in the sysfs path and the "delete
> > ringbuffers" path, neither of which is performance or concurrency sensitive. 
> > There's no change to any path that reads or writes data from/to the ring
> > buffers.  It seems like the mutex is the most straightforward solution to
> > preventing sysfs from accessing the ring buffer info while the memory is
> > being freed as part of "delete ringbuffers".
> > 
> > Michael  


I have no problems with the patch you did.
My discussion was more around the general issues with ringbuffers being detached
from the device. Not sure if it was even a good design choice but that is
something that is hard to fix now.