When cpu_stop_queue_two_works() begins to wake the stopper threads, it does so without preemption disabled, which leads to the following race condition:

The source CPU calls cpu_stop_queue_two_works(), with cpu1 as the source CPU, and cpu2 as the destination CPU. When adding the stopper threads to the wake queue used in this function, the source CPU stopper thread is added first, and the destination CPU stopper thread is added last.

When wake_up_q() is invoked to wake the stopper threads, the threads are woken up in the order that they are queued in, so the source CPU's stopper thread is woken up first, and it preempts the thread running on the source CPU.

The stopper thread will then execute on the source CPU, disable preemption, and begin executing multi_cpu_stop(), and wait for an ack from the destination CPU's stopper thread, with preemption still disabled. Since the worker thread that woke up the stopper thread on the source CPU is affine to the source CPU, and preemption is disabled on the source CPU, that thread will never run to dequeue the destination CPU's stopper thread from the wake queue, and thus, the destination CPU's stopper thread will never run, causing the source CPU's stopper thread to wait forever, and stall.

Disable preemption when waking the stopper threads in cpu_stop_queue_two_works().

Fixes: 0b26351b910f ("stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock") Co-Developed-by: Prasad Sodagudi psodagud@codeaurora.org Co-Developed-by: Pavankumar Kondeti pkondeti@codeaurora.org Signed-off-by: Isaac J. Manjarres isaacm@codeaurora.org Signed-off-by: Prasad Sodagudi psodagud@codeaurora.org Signed-off-by: Pavankumar Kondeti pkondeti@codeaurora.org Cc: stable@vger.kernel.org --- kernel/stop_machine.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index f89014a..1ff523d 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -270,7 +270,11 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, goto retry; }

- wake_up_q(&wakeq); + if (!err) { + preempt_disable(); + wake_up_q(&wakeq); + preempt_enable(); + }

return err; }