Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context

[Andrew Morton]

On Fri, 21 Aug 2020 11:08:48 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Fri, 21 Aug 2020 12:25:33 +0200
> Joerg Vehlow <lkml@xxxxxxxxxxx> 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...

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