Re: Locking between vfio hot-remove and pci sysfs sriov_numvfs

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On Fri, Dec 08, 2023 at 08:09:34PM +0000, Jim Harris wrote:
> On Fri, Dec 08, 2023 at 03:41:59PM -0400, Jason Gunthorpe wrote:
> > On Fri, Dec 08, 2023 at 05:07:22PM +0000, Jim Harris wrote:
> > > 
> > > Maybe for now we just whack this specific mole with a separate mutex
> > > for synchronizing access to sriov->num_VFs in the sysfs paths?
> > > Something like this (tested on my system):
> > 
> > TBH, I don't have the time right now to unpack this locking
> > mystery. Maybe Leon remembers?
> > 
> > device_lock() gets everywhere and does a lot of different stuff, so I
> > would be surprised if it was so easy..
> 
> The store() side still keeps the device_lock(), it just also acquires this
> new sriov lock. So store() side should observe zero differences. The only
> difference is now the show() side can acquire just the more-granular lock,
> since it is only trying to synchronize on sriov->num_VFs with the store()
> side. But maybe I'm missing something subtle here...

Oh if that is the only goal then probably a READ_ONCE is fine

Jason




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