On Fri, 21 Aug 2020 13:47:53 -0700 Andrew Morton <a...@linux-foundation.org> wrote:
> On Fri, 21 Aug 2020 11:08:48 -0400 Steven Rostedt <rost...@goodmis.org> wrote: > > > On Fri, 21 Aug 2020 12:25:33 +0200 > > Joerg Vehlow <l...@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