Re: [PATCH] kexec: change locking mechanism to a mutex

[Eric DeVolder]

On 9/22/23 11:28, Valentin Schneider wrote:
> On 21/09/23 17:59, Eric DeVolder wrote:
> > The design decision to use the atomic lock is described in the comment
> > from kexec_internal.h, cited above. However, examining the code of
> > __crash_kexec():
> >
> >          if (kexec_trylock()) {
> >                  if (kexec_crash_image) {
> >                          ...
> >                  }
> >                  kexec_unlock();
> >          }
> >
> > reveals that the use of kexec_trylock() here is actually a "best effort"
> > due to the atomic lock.  This atomic lock, prior to crash hotplug,
> > would almost always be assured (another kexec syscall could hold the lock
> > and prevent this, but that is about it).
> >
> > So at the point where the capture kernel would be invoked, if the lock
> > is not obtained, then kdump doesn't occur.
> >
> > It is possible to instead use a mutex with proper waiting, and utilize
> > mutex_trylock() as the "best effort" in __crash_kexec(). The use of a
> > mutex then avoids all the lock acquisition problems that were revealed
> > by the crash hotplug activity.
>
> @Dave thanks for the Cc, I'd have missed this otherwise.
>
>
> Prior to the atomic thingie, we actually had a mutex and did
> mutex_trylock() in __crash_kexec(). I'm a bit confused as this looks like a
> revert of
>    05c6257433b7 ("panic, kexec: make __crash_kexec() NMI safe")
> with just the helpers kept in - this doesn't seem to address any of the
> original issues regarding NMIs?
>
> Sebastian raised some good points in [1] regarding these issues.
> The main hurdle pointed out there is, if we end up in the slowpath during
> the unlock, then we can can up acquiring the ->wait_lock which isn't NMI
> safe.
>
> This is even worse on PREEMPT_RT, as both trylock and the unlock can end up
> acquiring the ->wait_lock.
>
> [1]: https://lore.kernel.org/all/YqyZ%2FUf14qkYtMDX@xxxxxxxxxxxxx/
Having reviewed the references, it would seem that Baoquan's approach of a new
lock to handle the hotplug activity is the way to go?
Eric