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

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