Messages in this thread | | | Date | Mon, 7 Nov 2022 16:10:34 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: Crash with PREEMPT_RT on aarch64 machine |
| |
+ 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.
> Honza >
Sebastian
| |