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Date:   Fri, 21 Aug 2020 11:08:48 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Joerg Vehlow <lkml@jv-coder.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Huang Ying <ying.huang@intel.com>,
        linux-kernel@vger.kernel.org,
        Joerg Vehlow <joerg.vehlow@aox-tech.de>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in
 interrupt context
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On Fri, 21 Aug 2020 12:25:33 +0200
Joerg Vehlow <lkml@jv-coder.de> wrote:

> Hi Andrew and Others (please read at least the part with @RT developers),
> 
> > Yup, mutex_trylock() from interrupt is improper.  Well dang, that's a
> > bit silly.  Presumably the 2006 spin_lock_mutex() wasn't taken with
> > irqs-off.
> >
> > Ho hum, did you look at switching the kexec code back to the xchg
> > approach?
> >  
> I looked into reverting to the xchg approach, but that seems to be
> not a good solution anymore, because the mutex is used in many places,
> a lot with waiting locks and I guess that would require spinning now,
> if we do this with bare xchg.
> 
> Instead I thought about using a spinlock, because they are supposed
> to be used in interrupt context as well, if I understand the documentation
> correctly ([1]).
> @RT developers
> Unfortunately the rt patches seem to interpret it a bit different and
> spin_trylock uses __rt_mutex_trylock again, with the same consequences as
> with the current code.
> 
> I tried raw_spinlocks, but it looks like they result in a deadlock at
> least in the rt kernel. Thiy may be because of memory allocations in the
> critical sections, that are not allowed if I understand it correctly.
> 
> I have no clue how to fix it at this point.
> 
> Jörg
> 
> [1] https://kernel.readthedocs.io/en/sphinx-samples/kernel-locking.html

There's only two places that wait on the mutex, and all other places
try to get it, and if it fails, it simply exits.

What I would do is introduce a kexec_busy counter, and have something
like this:

For the two locations that actually wait on the mutex:

loop:
	mutex_lock(&kexec_mutex);
	ret = atomic_inc_return(&kexec_busy);
	if (ret > 1) {
		/* Atomic context is busy on this counter, spin */
		atomic_dec(&kexec_busy);
		mutex_unlock(&kexec_mutex);
		goto loop;
	}
	[..]
	atomic_dec(&kexec_busy);
	mutex_unlock(&kexec_mutex);

And then all the other places that do the trylock:

	cant_sleep();
	ret = atomic_inc_return(&kexec_busy);
	if (ret > 1) {
		atomic_dec(&kexec_busy);
		return;
	}
	[..]
	atomic_dec(&kexec_busy);


-- Steve