From: Prasad Sodagudi psodagud@codeaurora.org

When cpu_stop_queue_work() releases the lock for the stopper thread that was queued into its wake queue, preemption is enabled, which leads to the following deadlock:

CPU0 CPU1 sched_setaffinity(0, ...) __set_cpus_allowed_ptr() stop_one_cpu(0, ...) stop_two_cpus(0, 1, ...) cpu_stop_queue_work(0, ...) cpu_stop_queue_two_works(0, ..., 1, ...)

-grabs lock for migration/0- -spins with preemption disabled, waiting for migration/0's lock to be released-

-adds work items for migration/0 and queues migration/0 to its wake_q-

-releases lock for migration/0 and preemption is enabled-

-current thread is preempted, and __set_cpus_allowed_ptr has changed the thread's cpu allowed mask to CPU1 only-

-acquires migration/0 and migration/1's locks-

-adds work for migration/0 but does not add migration/0 to wake_q, since it is already in a wake_q-

-adds work for migration/1 and adds migration/1 to its wake_q-

-releases migration/0 and migration/1's locks, wakes migration/1, and enables preemption-

-since migration/1 is requested to run, migration/1 begins to run and waits on migration/0, but migration/0 will never be able to run, since the thread that can wake it is affine to CPU1-

Disable preemption in cpu_stop_queue_work() before queueing works for stopper threads, and queueing the stopper thread in the wake queue, 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: Isaac J. Manjarres isaacm@codeaurora.org Signed-off-by: Prasad Sodagudi psodagud@codeaurora.org Signed-off-by: Isaac J. Manjarres isaacm@codeaurora.org Cc: stable@vger.kernel.org --- kernel/stop_machine.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index 34b6652..067cb83 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work) unsigned long flags; bool enabled;

+ preempt_disable(); raw_spin_lock_irqsave(&stopper->lock, flags); enabled = stopper->enabled; if (enabled) @@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work) raw_spin_unlock_irqrestore(&stopper->lock, flags);

wake_up_q(&wakeq); + preempt_enable();

return enabled; }