Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock | From | Jürgen Groß <> | Date | Tue, 17 Dec 2019 18:10:19 +0100 |
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On 17.12.19 17:24, SeongJae Park wrote: > On Tue, 17 Dec 2019 17:13:42 +0100 "Jürgen Groß" <jgross@suse.com> wrote: > >> On 17.12.19 17:07, SeongJae Park wrote: >>> From: SeongJae Park <sjpark@amazon.de> >>> >>> '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@amazon.de> >>> --- >>> 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
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