cpu stopper threads and setaffinity leads to deadlock

[Sodagudi Prasad]

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
https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1740526.html 
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
    Stack:
    [<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 cpu7âs execution, stress-ng 
will never run on cpu7, and cpu3âs migration task is never woken up.

Essentially:
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-affinâs 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)
                cpu_stop_signal_done(work->done);
-       raw_spin_unlock_irqrestore(&stopper->lock, flags);

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


-Thanks, Prasad

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