Re: [PATCH v3] hwrng: core: Freeze khwrng thread during suspend

[Maciej S. Szmigiero]

On 05.11.2019 01:24, Stephen Boyd wrote:
> Quoting Maciej S. Szmigiero (2019-10-29 08:50:52)
>> On 29.10.2019 15:44, Stephen Boyd wrote:
>>> Quoting Maciej S. Szmigiero (2019-10-28 16:45:31)
>>>> Hi Stephen,
>>>>
>>>> On 06.08.2019 01:32, Stephen Boyd wrote:
>>>>> The hwrng_fill() function can run while devices are suspending and
>>>>> resuming. If the hwrng is behind a bus such as i2c or SPI and that bus
>>>>> is suspended, the hwrng may hang the bus while attempting to add some
>>>>> randomness. It's been observed on ChromeOS devices with suspend-to-idle
>>>>> (s2idle) and an i2c based hwrng that this kthread may run and ask the
>>>>> hwrng device for randomness before the i2c bus has been resumed.
>>>>>
>>>>> Let's make this kthread freezable so that we don't try to touch the
>>>>> hwrng during suspend/resume. This ensures that we can't cause the hwrng
>>>>> backing driver to get into a bad state because the device is guaranteed
>>>>> to be resumed before the hwrng kthread is thawed.
>>>>
>>>> This patch broke suspend with virtio-rng loaded (it hangs).
>>>>
>>>> The problematic call chain is:
>>>> virtrng_freeze() -> remove_common() -> hwrng_unregister() ->
>>>> kthread_stop().
>>>>
>>>> It looks like kthread_stop() can't finish on a frozen khwrng thread.
>>>
>>> Can you provide the suspend/resume logs?
>>
>> There isn't much in the kernel log, the closest thing I can get is
>> with dyndbg="file drivers/base/power/main.c +p":
>> [   58.441073][ T3511] virtio-pci 0000:00:06.0: bus freeze
>> [   58.448744][ T3511] virtio-pci 0000:00:05.0: bus freeze
>> [   58.454500][ T3511] virtio-pci 0000:00:04.0: bus freeze
>> [   58.456873][ T3511] virtio-pci 0000:00:03.0: bus freeze
>>
>> And then the VM hangs.
>>
>> The 0000:00:03.0 pci device is virtio-rng.
>>
>> If I add printks around that kthread_stop() in hwrng_unregister()
>> only the first one gets printed.
> 
> Ok. I don't know why virtio rng wants to remove itself and then reprobe
> across suspend/resume. Do you know the history there?

Hard to tell, I have added Amit, who had implemented these PM callbacks
back in 2012, to CC now.

> Can you enable the dynamic debug printk in __refrigerator()?
> 
> 	file kernel/freezer.c +p
> 
> That will let us know when the kthread has been frozen/thawed.
> 
> Either way, it sounds like maybe it's what you say, virtio rng wants to
> call kthread_stop() on a kthread that's been frozen already and then it
> just hangs waiting for the thread to wake up, which it never does. I
> can't convince myself that the schedule() inside __refrigerator() won't
> wake up though. I would think it leaves the refrigerator when
> kthread_stop() is called because the kthread will wakeup from
> wake_up_process() in kthread_stop(), see it should stop in
> __refrigerator() and eventually exit. Maybe the hwrng thread is stuck
> somewhere else?
> 

Yes, it turns out the hwrng kthread is actually stuck inside
add_hwgenerator_randomness() in wait_event_freezable() call introduced
by commit 59b569480dc8
("random: Use wait_event_freezable() in add_hwgenerator_randomness()").

wait_event_freezable() ultimately calls __refrigerator() with its
check_kthr_stop argument set to false, which causes it to keep the
kthread frozen even if somebody calls kthread_stop() on it.

Calling wait_event_freezable() with kthread_should_stop() as a condition
seems racy because it doesn't take into account the situation where this
condition becomes true on a kthread marked for freezing only after it
has been checked.

I was able to make the VM write a s2disk image with the following change:
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2500,8 +2500,8 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	 * We'll be woken up again once below random_write_wakeup_thresh,
 	 * or when the calling thread is about to terminate.
 	 */
-	wait_event_freezable(random_write_wait,
-			kthread_should_stop() ||
+	wait_event_interruptible(random_write_wait,
+			kthread_should_stop() || freezing(current) ||
 			ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
 	mix_pool_bytes(poolp, buffer, count);
 	credit_entropy_bits(poolp, entropy);

Calling freezing() should avoid the issue that the commit 59b569480dc8
has fixed, as it is only a checking function, it doesn't actually do
the freezing.

However, while the written image seems valid (the machine will resume
successfully from it) the suspend process still hangs, only now a bit
later.

It turns out there is a second issue where the set_freezable() call
at the beginning of hwrng_fillfn() will freeze this kthread with
(again) check_kthr_stop argument set to false when this kthread gets
relaunched when devices are resumed to write the hibernation image
at the suspend time.

That makes the frozen kthread impossible to stop on shutdown, so the
VM hangs there.

If I only clear the PF_NOFREEZE flag instead of calling set_freezable()
at the beginning of hwrng_fillfn() the suspend process will complete
successfully.

However, it seems to me that the real solution here would be to
change the virtio-rng driver to not unregister / reregister itself
on PM events rather than fight the freezer / kthread_stop()
interactions.

Maciej