Re: [Xen-devel] [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[JÃrgen GroÃ]

On 17.12.19 17:24, SeongJae Park wrote:
> On Tue, 17 Dec 2019 17:13:42 +0100 "JÃrgen GroÃ" <jgross@xxxxxxxx> wrote:
>
> > On 17.12.19 17:07, SeongJae Park wrote:
> > > From: SeongJae Park <sjpark@xxxxxxxxx>
> > >
> > > 'reclaim_memory' callback can race with a driver code as this callback
> > > will be called from any memory pressure detected context.  To deal with
> > > the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
> > > 'reclaim_memory' callback is called, the lock of the device which passed
> > > to the callback as its argument is locked.  Thus, drivers registering
> > > their 'reclaim_memory' callback should protect the data that might race
> > > with the callback with the lock by themselves.
> > >
> > > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> > > ---
> > >    drivers/xen/xenbus/xenbus_probe.c         |  1 +
> > >    drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
> > >    include/xen/xenbus.h                      |  2 ++
> > >    3 files changed, 11 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> > > index 5b471889d723..b86393f172e6 100644
> > > --- a/drivers/xen/xenbus/xenbus_probe.c
> > > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > > @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
> > >    		goto fail;
> > >    
> > >    	dev_set_name(&xendev->dev, "%s", devname);
> > > +	spin_lock_init(&xendev->reclaim_lock);
> > >    
> > >    	/* Register with generic device framework. */
> > >    	err = device_register(&xendev->dev);
> > > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > > index 7e78ebef7c54..516aa64b9967 100644
> > > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > > @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> > >    static int backend_reclaim_memory(struct device *dev, void *data)
> > >    {
> > >    	const struct xenbus_driver *drv;
> > > +	struct xenbus_device *xdev;
> > > +	unsigned long flags;
> > >    
> > >    	if (!dev->driver)
> > >    		return 0;
> > >    	drv = to_xenbus_driver(dev->driver);
> > > -	if (drv && drv->reclaim_memory)
> > > -		drv->reclaim_memory(to_xenbus_device(dev));
> > > +	if (drv && drv->reclaim_memory) {
> > > +		xdev = to_xenbus_device(dev);
> > > +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);
> >
> > You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
> > too? I can't see a reason why you'd want to disable irqs here.
>
> I needed to diable irq here as this is called from the memory shrinker context.

Okay.

> Also, used 'trylock' because the 'probe()' and 'remove()' code of the driver
> might include memory allocation.  And the xen-blkback actually does.  If the
> allocation shows a memory pressure during the allocation, it will trigger this
> shrinker callback again and then deadlock.

In that case you need to either return when you didn't get the lock or

- when obtaining the lock during probe() and remove() set a variable
  containing the current cpu number
- and reset that to e.g NR_CPUS before releasing the lock again
- in the shrinker callback do trylock, and if you didn't get the lock
  test whether the cpu-variable above is set to your current cpu and
  continue only if yes; if not, redo the the trylock


Juergen