Date:   Wed, 9 Nov 2022 15:21:00 +0100
From:   Pierre Gondois <pierre.gondois@....com>
To:     Jan Kara <jack@...e.cz>, Mark Rutland <mark.rutland@....com>
Cc:     Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Crash with PREEMPT_RT on aarch64 machine



On 11/9/22 14:52, Pierre Gondois wrote:
> 
> 
> 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
> 

I misread the logs, the issue was not reproduced on the eMAG,
Sorry for the noise.

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