From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.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>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context
Date: Fri, 21 Aug 2020 13:47:53 -0700 [thread overview]
Message-ID: <20200821134753.9547695c9b782275be3c95b5@linux-foundation.org> (raw)
In-Reply-To: <20200821110848.6c3183d1@oasis.local.home>
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...
Sigh. Is it too hard to make mutex_trylock() usable from interrupt
context?
next prev parent reply other threads:[~2020-08-21 20:47 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-28 11:41 [BUG RT] dump-capture kernel not executed for panic in interrupt context Joerg Vehlow
2020-05-28 12:46 ` Steven Rostedt
2020-07-22 4:30 ` Joerg Vehlow
2020-07-22 20:51 ` Steven Rostedt
2020-07-27 23:36 ` Andrew Morton
2020-08-21 10:25 ` Joerg Vehlow
2020-08-21 15:08 ` Steven Rostedt
2020-08-21 20:47 ` Andrew Morton [this message]
2020-08-21 21:03 ` Steven Rostedt
2020-08-22 12:32 ` peterz
2020-08-22 23:49 ` Steven Rostedt
2020-09-07 11:41 ` peterz
2020-09-07 12:49 ` Valentin Schneider
2020-09-14 19:00 ` Steven Rostedt
2020-09-07 10:51 ` Joerg Vehlow
2020-09-07 11:46 ` peterz
2020-09-07 12:03 ` Joerg Vehlow
2020-09-07 16:23 ` peterz
2020-09-08 5:48 ` Joerg Vehlow
2020-09-09 5:46 ` Joerg Vehlow
2020-09-11 22:48 ` Eric W. Biederman
2020-09-14 6:03 ` Joerg Vehlow
2020-09-14 16:46 ` Eric W. Biederman
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