[Bug 200493] Killing pid 11 (idle) wedges the disk IO

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=200493

--- Comment #11 from commit-hook at freebsd.org ---
A commit references this bug:

Author: kib
Date: Fri May 29 16:26:08 UTC 2015
New revision: 283745
URL: https://svnweb.freebsd.org/changeset/base/283745

Log:
  When delivering a signal with default disposition to the thread,
  tdsigwakeup() increases the priority of the low-priority threads, to
  give them a chance to be terminated timely.  Also, kernel allows user
  to signal kernel processes.  The combined effect is that signalling
  idle process bump a priority of the selected delivery thread, which
  starts eating CPU.

  Check for the delivery thread be an idle thread and do not raise its
  priority then.

  The signal delivery to the kernel threads must be opt-in feature.
  Kernel thread should explicitely declare the ability to handle signals
  directed to it.  E.g., nfsd threads check for signal as an indication
  of exit request.

  Most threads do not handle signals at all, and queuing the signal to
  them causes odd side-effects.  Most innocent consequence is the memory
  leak due to queued ksiginfo, which is never deleted from the sigqueue.
  Code to prevent even queuing signals to the kernel threads is trivial,
  but it requires careful examination of each call to kproc/kthread
  creation to decide should the signalling be allowed.  The commit is a
  stop-gap measure which fixes the immediate case for now.

  PR:    200493
  Reported and tested by:    trasz
  Discussed with:    trasz, emaste
  Sponsored by:    The FreeBSD Foundation
  MFC after:    1 week

Changes:
  head/sys/kern/kern_sig.c

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