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

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Date:   Thu, 28 Mar 2019 00:30:57 -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-hyperv@...r.kernel.org>,
        "linux-kernel@...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 21, 2019 at 04:04:20PM +0000, Michael Kelley wrote:
> From: Kimberly Brown <kimbrownkd@...il.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