[X][PATCH 0/4] LP#1821259 Fix for deadlock in cpu_stopper

[Mauricio Faria de Oliveira]

BugLink: https://bugs.launchpad.net/bugs/1821259

[Impact] 

 * This problem hard locks up 2 CPUs in a deadlock, and this
   soft locks up other CPUs as an effect; the system becomes
   unusable.

 * This is relatively rare / difficult to hit because it's a
   corner case in scheduling/load balancing that needs timing
   with CPU stopper code. And it needs SMP plus _NUMA_ system.
   (but it can be hit with synthetic test case attached in LP.)

 * Since SMP plus NUMA usually equals _servers_ it looks like
   a good idea to prevent this bug / hard lockups / rebooting.

 * The fix resolves the potential deadlock by removing one of
   the calls required to deadlock from under the locked code.

[Test Case]

 * There's a synthetic test case to reproduce this problem
   (although without the stack traces - just a system hang)
   attached to this LP bug.

 * It uses kprobes/mdelay/cpu stopper calls to force the code
   to execute and force the timing/locking condition to occur.

 * $ sudo insmod kmod-stopper.ko 

   Some dmesg logging occurs, and systems either hangs or not.
   See examples in comments.
   
[Regression Potential] 

 * These are patches to the cpu stop_machine.c code, and they
   change a bit how it works;  however, there are no upstream
   fixes for these patches anymore and they are still the top
   of the 'git log --oneline -- kernel/stop_machine.c' output.

 * These patches have been verified with the synthetic test case
   and 'stress-ng --class scheduler --sequential 0' (no regressions)
   on guest with 2 CPUs and one physical system with 24 CPUs.

[Other Info]
 
 * The patches are required on Xenial and later.
 * There are 4 patches for Xenial, and 2 patches pending for Bionic.
 * All patches are applied from Cosmic onwards.

Isaac J. Manjarres (2):
  stop_machine: Disable preemption when waking two stopper threads
  stop_machine: Disable preemption after queueing stopper threads

Peter Zijlstra (1):
  stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock

Prasad Sodagudi (1):
  stop_machine: Atomically queue and wake stopper threads

 kernel/stop_machine.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

-- 
2.17.1