Re: Crash with PREEMPT_RT on aarch64 machine

[Pierre Gondois <pierre.gondois@arm.com>]

On 11/9/22 12:01, Jan Kara wrote:
> On Wed 09-11-22 09:55:07, Mark Rutland wrote:
>> On Tue, Nov 08, 2022 at 06:45:29PM +0100, Jan Kara wrote:
>>> On Tue 08-11-22 10:53:40, Mark Rutland wrote:
>>>> On Mon, Nov 07, 2022 at 11:49:01AM -0500, Waiman Long wrote:
>>>>> On 11/7/22 10:10, Sebastian Andrzej Siewior wrote:
>>>>>> + locking, arm64
>>>>>>
>>>>>> On 2022-11-07 14:56:36 [+0100], Jan Kara wrote:
>>>>>>>> spinlock_t and raw_spinlock_t differ slightly in terms of locking.
>>>>>>>> rt_spin_lock() has the fast path via try_cmpxchg_acquire(). If you
>>>>>>>> enable CONFIG_DEBUG_RT_MUTEXES then you would force the slow path which
>>>>>>>> always acquires the rt_mutex_base::wait_lock (which is a raw_spinlock_t)
>>>>>>>> while the actual lock is modified via cmpxchg.
>>>>>>> So I've tried enabling CONFIG_DEBUG_RT_MUTEXES and indeed the corruption
>>>>>>> stops happening as well. So do you suspect some bug in the CPU itself?
>>>>>> If it is only enabling CONFIG_DEBUG_RT_MUTEXES (and not whole lockdep)
>>>>>> then it looks very suspicious.
>>>>>> CONFIG_DEBUG_RT_MUTEXES enables a few additional checks but the main
>>>>>> part is that rt_mutex_cmpxchg_acquire() + rt_mutex_cmpxchg_release()
>>>>>> always fail (and so the slowpath under a raw_spinlock_t is done).
>>>>>>
>>>>>> So if it is really the fast path (rt_mutex_cmpxchg_acquire()) then it
>>>>>> somehow smells like the CPU is misbehaving.
>>>>>>
>>>>>> Could someone from the locking/arm64 department check if the locking in
>>>>>> RT-mutex (rtlock_lock()) is correct?
>>>>>>
>>>>>> rtmutex locking uses try_cmpxchg_acquire(, ptr, ptr) for the fastpath
>>>>>> (and try_cmpxchg_release(, ptr, ptr) for unlock).
>>>>>> Now looking at it again, I don't see much difference compared to what
>>>>>> queued_spin_trylock() does except the latter always operates on 32bit
>>>>>> value instead a pointer.
>>>>>
>>>>> Both the fast path of queued spinlock and rt_spin_lock are using
>>>>> try_cmpxchg_acquire(), the only difference I saw is the size of the data to
>>>>> be cmpxchg'ed. qspinlock uses 32-bit integer whereas rt_spin_lock uses
>>>>> 64-bit pointer. So I believe it is more on how the arm64 does cmpxchg. I
>>>>> believe there are two different ways of doing it depending on whether LSE
>>>>> atomics is available in the platform. So exactly what arm64 system is being
>>>>> used here and what hardware capability does it have?
>>>>
>>>>  From the /proc/cpuinfo output earlier, this is a Neoverse N1 system, with the
>>>> LSE atomics. Assuming the kernel was built with support for atomics in-kernel
>>>> (which is selected by default), it'll be using the LSE version.
>>>
>>> So I was able to reproduce the corruption both with LSE atomics enabled &
>>> disabled in the kernel. It seems the problem takes considerably longer to
>>> reproduce with LSE atomics enabled but it still does happen.
>>>
>>> BTW, I've tried to reproduced the problem on another aarch64 machine with
>>> CPU from a different vendor:
>>>
>>> processor       : 0
>>> BogoMIPS        : 200.00
>>> Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma dcpop asimddp asimdfhm
>>> CPU implementer : 0x48
>>> CPU architecture: 8
>>> CPU variant     : 0x1
>>> CPU part        : 0xd01
>>> CPU revision    : 0
>>>
>>> And there the problem does not reproduce. So might it be a genuine bug in
>>> the CPU implementation?
>>
>> Perhaps, though I suspect it's more likely that we have an ordering bug in the
>> kernel code, and it shows up on CPUs with legitimate but more relaxed ordering.
>> We've had a couple of those show up on Apple M1, so it might be worth trying on
>> one of those.
>>
>> How easy is this to reproduce? What's necessary?
> 
> As Pierre writes, on Ampere Altra machine running dbench benchmark on XFS
> filesystem triggers this relatively easily (it takes it about 10 minutes to
> trigger without atomics and about 30 minutes to trigger with the atomics
> enabled).
> 
> Running the benchmark on XFS somehow seems to be important, we didn't see
> the crash happen on ext4 (which may just mean it is less frequent on ext4
> and didn't trigger in our initial testing after which we've started to
> investigate crashes with XFS).
> 
> 								Honza

It was possible to reproduce on an Ampere eMAG. It takes < 1min to reproduce
once dbench is launched and seems more likely to trigger with the previous diff
applied. It even sometimes triggers without launching dbench on the Altra.

/proc/cpuinfo for eMAG:
   processor       : 0
   BogoMIPS        : 80.00
   Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
   CPU implementer : 0x50
   CPU architecture: 8
   CPU variant     : 0x3
   CPU part        : 0x000
   CPU revision    : 2