From: Mark Rutland <mark.rutland@arm.com>
To: Waiman Long <longman@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Jan Kara <jack@suse.cz>, 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>,
Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: Crash with PREEMPT_RT on aarch64 machine
Date: Tue, 8 Nov 2022 10:53:40 +0000 [thread overview]
Message-ID: <Y2o1NAE7d6Tf5ILt@FVFF77S0Q05N.cambridge.arm.com> (raw)
In-Reply-To: <359cc93a-fce0-5af2-0fd5-81999fad186b@redhat.com>
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.
Mark.
next prev parent reply other threads:[~2022-11-08 10:53 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-03 11:54 Crash with PREEMPT_RT on aarch64 machine Jan Kara
2022-11-04 16:30 ` Sebastian Andrzej Siewior
2022-11-07 13:56 ` Jan Kara
2022-11-07 15:10 ` Sebastian Andrzej Siewior
2022-11-07 16:30 ` Jan Kara
2022-11-07 17:12 ` Sebastian Andrzej Siewior
2022-11-07 16:49 ` Waiman Long
2022-11-08 10:53 ` Mark Rutland [this message]
2022-11-08 17:45 ` Jan Kara
2022-11-09 9:55 ` Mark Rutland
2022-11-09 10:11 ` Pierre Gondois
2022-11-09 10:54 ` Jan Kara
2022-11-09 11:01 ` Jan Kara
2022-11-09 13:52 ` Pierre Gondois
2022-11-09 14:21 ` Pierre Gondois
2022-11-09 12:57 ` Will Deacon
2022-11-09 15:40 ` Jan Kara
2022-11-11 14:27 ` Jan Kara
2022-11-14 12:41 ` Will Deacon
2022-11-28 15:58 ` Sebastian Andrzej Siewior
2022-11-28 20:30 ` kernel test robot
2022-11-28 21:11 ` kernel test robot
2022-11-29 5:16 ` kernel test robot
2022-11-29 5:26 ` kernel test robot
2022-11-29 6:48 ` kernel test robot
2022-11-29 7:39 ` kernel test robot
2022-11-30 17:20 ` Pierre Gondois
2022-12-01 12:37 ` Jan Kara
2022-11-30 20:22 ` Mel Gorman
2022-12-01 17:09 ` Mel Gorman
[not found] ` <20221104080637.626-1-hdanton@sina.com>
2022-11-07 12:41 ` Jan Kara
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