linux-kernel - Re: Crash with PREEMPT

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Date:   Mon, 7 Nov 2022 18:12:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Jan Kara <jack@...e.cz>
Cc:     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>,
        Waiman Long <longman@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Crash with PREEMPT_RT on aarch64 machine

On 2022-11-07 17:30:16 [+0100], Jan Kara wrote:
> On Mon 07-11-22 16:10:34, 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. 
> 
> Just to confirm, CONFIG_DEBUG_RT_MUTEXES is the only thing I've enabled and
> the list corruption disappeared.

I don't know if this works but:
if you tell task_struct_cachep to be created with SLAB_CACHE_DMA32 then
the pointer should only have the lower 32bit set. With this could make
rt_mutex_base::owner an atomic_t type. You could then replace
try_cmpxchg_acquire() with atomic_try_cmpxchg_acquire() and do the 32bit
cmpxchg. You would then need set the const upper 32bit of the pointer
while returning the pointer.
I have no idea how much sense it makes but you would avoid the 64bit
cmpxchg making those two a little more alike :)

> 								Honza
> 

Sebastian