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Date:   Fri, 21 Aug 2020 17:03:34 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Joerg Vehlow <lkml@jv-coder.de>,
        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>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in
 interrupt context
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On Fri, 21 Aug 2020 13:47:53 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 21 Aug 2020 11:08:48 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > 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);  
> 
> Aw gee.  Hide all this in include/linux/rostedt_lock.h...

Heh, if this was the way to go, I would have definitely recommended
packaging that up in static inline functions in some local header. Not
necessarily rostedt_lock.h, but I'll use that if people let me :-)

> 
> Sigh.  Is it too hard to make mutex_trylock() usable from interrupt
> context?


That's a question for Thomas and Peter Z.

-- Steve