After cpu_stop_queue_two_works() queues the cpu_stop works for the stopper threads, it releases the locks held for both threads, which enables preemption, which allows the following race condition to occur:

On one CPU, call it CPU 3, thread 1 invokes cpu_stop_queue_two_works(2, 3,...), and the execution is such that thread 1 queues the works for migration/2 and migration/3, and is preempted after releasing the locks for migration/2 and migration/3, but before waking the threads.

Then, On CPU 2, a kworker, call it thread 2, is running, and it invokes cpu_stop_queue_two_works(1, 2,...), such that thread 2 queues the works for migration/1 and migration/2. Meanwhile, on CPU 3, thread 1 resumes execution, and wakes migration/2 and migration/3. This means that when CPU 2 releases the locks for migration/1 and migration/2, but before it wakes those threads, it can be preempted by migration/2.

If thread 2 is preempted by migration/2, then migration/2 will execute the first work item successfully, since migration/3 was woken up by CPU 3, but when it goes to execute the second work item, it disables preemption, calls multi_cpu_stop(), and thus, CPU 2 will wait forever for migration/1, which should have been woken up by thread 2. However migration/1 cannot be woken up by thread 2, since it is a kworker, so it is affine to CPU 2, but CPU 2 is running migration/2 with preemption disabled, so thread 2 will never run.

Disable preemption after queueing works for stopper threads to ensure that the operation of queueing the works and waking the stopper threads is atomic.

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 | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index f89014a..e190d1e 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -260,6 +260,15 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, err = 0; __cpu_stop_queue_work(stopper1, work1, &wakeq); __cpu_stop_queue_work(stopper2, work2, &wakeq); + /* + * The waking up of stopper threads has to happen + * in the same scheduling context as the queueing. + * Otherwise, there is a possibility of one of the + * above stoppers being woken up by another CPU, + * and preempting us. This will cause us to n ot + * wake up the other stopper forever. + */ + preempt_disable(); unlock: raw_spin_unlock(&stopper2->lock); raw_spin_unlock_irq(&stopper1->lock); @@ -270,7 +279,10 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, goto retry; }

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

return err; }