[PATCH 0/4] Signal: Fix hard lockup problem in flush_sigqueue()

[Waiman Long <longman@redhat.com>]

It was found that if a process has accumulated sufficient number of
pending signals, the exiting of that process may cause its parent to
have hard lockup when running on a debug kernel with a slow memory
freeing path (like with KASAN enabled).

  release_task() => flush_sigqueue()

The lockup condition can be reproduced on a large system with a lot of
memory and relatively slow CPUs running LTP's sigqueue_9-1 test on a
debug kernel.

This patchset tries to mitigate this problem by introducing a new kernel
memory freeing queue mechanism modelled after the wake_q mechanism for
waking up tasks. Then flush_sigqueue() and release_task() are modified
to use the freeing queue mechanism to defer the actual memory object
freeing until after releasing the tasklist_lock and with irq re-enabled.

With the patchset applied, the hard lockup problem was no longer
reproducible on the debug kernel.

Waiman Long (4):
  mm: Implement kmem objects freeing queue
  signal: Make flush_sigqueue() use free_q to release memory
  signal: Add free_uid_to_q()
  mm: Do periodic rescheduling when freeing objects in kmem_free_up_q()

 include/linux/sched/user.h |  3 +++
 include/linux/signal.h     |  4 ++-
 include/linux/slab.h       | 28 +++++++++++++++++++++
 kernel/exit.c              | 12 ++++++---
 kernel/signal.c            | 29 +++++++++++++---------
 kernel/user.c              | 17 ++++++++++---
 mm/slab_common.c           | 50 ++++++++++++++++++++++++++++++++++++++
 security/selinux/hooks.c   |  8 ++++--
 8 files changed, 128 insertions(+), 23 deletions(-)

-- 
2.18.1