[PATCH v2] stop_machine: Disable preemption when waking two stopper threads

[Isaac J. Manjarres]

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() to ensure that the worker thread
that is waking up the stopper threads isn't preempted
by the source CPU's stopper thread, and permanently
scheduled out, leaving the remaining stopper thread asleep
in the wake queue.

Co-developed-by: Pavankumar Kondeti <pkondeti@xxxxxxxxxxxxxx>
Signed-off-by: Prasad Sodagudi <psodagud@xxxxxxxxxxxxxx>
Signed-off-by: Pavankumar Kondeti <pkondeti@xxxxxxxxxxxxxx>
Signed-off-by: Isaac J. Manjarres <isaacm@xxxxxxxxxxxxxx>
---
 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;
 }
-- 
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