Hi all,
Are there any comments about this patch?
Thanks, Isaac Manjarres On 2018-07-17 12:35, Isaac J. Manjarres wrote:
This commit:
9fb8d5dc4b64 ("stop_machine, Disable preemption when waking two stopper threads")
does not fully address the race condition that can occur as follows:
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: 9fb8d5dc4b64 ("stop_machine, Disable preemption when waking two stopper threads") 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 | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index 1ff523d..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); @@ -271,7 +280,6 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, }
if (!err) {
preempt_disable();