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 <psoda...@codeaurora.org>
Co-Developed-by: Pavankumar Kondeti <pkond...@codeaurora.org>
Signed-off-by: Isaac J. Manjarres <isa...@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psoda...@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkond...@codeaurora.org>
Cc: sta...@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();
wake_up_q(&wakeq);
preempt_enable();
}
