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Date:   Mon, 7 Nov 2022 16:10:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Jan Kara <jack@suse.cz>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: Crash with PREEMPT_RT on aarch64 machine
Message-ID: <Y2kf6tcX47Cl7q0W@linutronix.de>
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 <Y2U+Je+LICO2HkNY@linutronix.de>
 <20221107135636.biouna36osqc4rik@quack3>
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+ 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