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Subjectcpu stopper threads and setaffinity leads to deadlock
Hi Peter and Tglx,

We are observing another deadlock issue due to commit
0b26351b91(stop_machine, sched: Fix migrate_swap() vs. active_balance()
deadlock), even after taking the following fix
on the Linux-4.14.56 kernel.

Here is the scenario that leads to this deadlock.
We have used the stress-ng-64 --affinity test case to reproduce this
issue in a controlled environment, while simultaneously running CPU hot
plug and task migrations.

Stress-ng-affin (call stack shown below) is changing its own affinity
from cpu3 to cpu7. Stress-ng-affin is preempted in the
cpu_stop_queue_work() function
as soon as the stopper lock for migration/3 is released . At the same
time, on CPU 7, cross migration of tasks happens between cpu3 and cpu7.

Process: stress-ng-affin, cpu: 3 pid: 1748 start: 0xffffffd8817e4480
Task name: stress-ng-affin pid: 1748 cpu: 3 start: ffffffd8817e4480
state: 0x0 exit_state: 0x0 stack base: 0xffffff801c8e8000 Prio: 120
[<ffffff87754864f4>] __switch_to+0xb8
[<ffffff87763ebf8c>] __schedule+0x690
[<ffffff87763ec388>] preempt_schedule_common+0x100
[<ffffff87763eb8f4>] preempt_schedule+0x24
[<ffffff87763f0e58>] _raw_spin_unlock_irqrestore+0x64
[<ffffff8775574f8c>] cpu_stop_queue_work+0x9c
[<ffffff8775574dfc>] stop_one_cpu+0x58
[<ffffff87754e4884>] __set_cpus_allowed_ptr+0x234
[<ffffff87754e8888>] sched_setaffinity+0x150
[<ffffff87754e8ad8>] SyS_sched_setaffinity+0xcc
[<ffffff87754837c0>] el0_svc_naked+0x34
[<0>] UNKNOWN+0x0

Due to cross migration of tasks between cpu7 and cpu3, migration/7 has
started executing and waits for the migration/3 task, so that they can
proceed within the multi cpu stop state machine together.
Unfortunately stress-ng-affin is affine to cpu7, and since migration 7
has started running, and has monopolized cpu7’s execution, stress-ng
will never run on cpu7, and cpu3’s migration task is never woken up.

Due to the nature of the wake_q interface, a thread can only be in at
most one wake queue at a time.
migration/3 is currently in stress-ng-affin’s wake_q. This means that no
other thread can add migration/3 to their wake queue.
Thus, even if any attempt is made to stop CPU 3 (e.g. cross-migration,
hot plugging, etc), no thread will wake up migration/3.

Below change helped to fix this deadlock.
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index e190d1e..f932e1e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu,
struct cpu_stop_work *work)
__cpu_stop_queue_work(stopper, work, &wakeq);
else if (work->done)
- raw_spin_unlock_irqrestore(&stopper->lock, flags);

+ raw_spin_unlock_irqrestore(&stopper->lock, flags);

-Thanks, Prasad
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Linux Foundation Collaborative Project

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