Debian bug #46859, Debian bug #195360

IRC, unknown channel, unknown date:

<youpi> azeem, marcus: ext2fs.static: thread-cancel.c:55:
  hurd_thread_cancel: Assertion '! __spin_lock_locked
  (&ss->critical_section_lock)' failed
<youpi> I actually don't understand this assertion
<youpi> it's just before __spin_lock (&ss->critical_section_lock);
<youpi> why should one check that a lock is free before taking it ?
<youpi> just the same in hurdexec.c
<youpi> (no, ss is not our own sigstate, so it's not safe to assume no
  other path can take it)
<youpi> there's another one in sysdeps/mach/hurd/spawni.c
<youpi> and jmp-unwind.c
<antrik> youpi: why do you think it's nonsense?... the fact that we take
  the lock (so we can't be interrupted) doesn't mean we are willing to wait
  for others to release the lock... maybe the code path should never be
  reached while others have a lock, or something
<youpi> then it's useless to take the lock
<youpi> "we take the lock (so we can't be interrupted)": no, it's not _our_
  lock here, it's the lock of the thread we want to cancel
<antrik> what exactly is cancelling a thread?... (sorry, I don't really
  have experience with thread programming)
<youpi> ~= killing it
<antrik> well, we take the lock so nobody can mess with the thread while we
  are cancelling it, no?...
<youpi> yes
<youpi> that is fine
<youpi> but checking that the lock is free before taking it doesn't make
<youpi> why nobody should be able to take the lock ?
<youpi> and if nobody is, why do we take it ? (since nobody would be able
  to take it)
<antrik> well, maybe after taking the lock, we do some action that might
  result in others trying to take it...
<youpi> nope: look at the code :)
<youpi> or maybe the cancel_hook, but I really doubt it

See discussion about critical_section_lock on glibc.