Re: Crash with PREEMPT_RT on aarch64 machine

[Mark Rutland]

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?

Thanks,
Mark.