lkml.org 
[lkml]   [2018]   [Aug]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 4.14 008/104] stop_machine: Disable preemption after queueing stopper threads
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Isaac J. Manjarres <isaacm@codeaurora.org>

commit 2610e88946632afb78aa58e61f11368ac4c0af7b upstream.

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.

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bigeasy@linutronix.de
Cc: gregkh@linuxfoundation.org
Cc: matt@codeblueprint.co.uk
Fixes: 9fb8d5dc4b64 ("stop_machine, Disable preemption when waking two stopper threads")
Link: http://lkml.kernel.org/r/1531856129-9871-1-git-send-email-isaacm@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/stop_machine.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -260,6 +260,15 @@ retry:
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 @@ unlock:
}

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

\
 
 \ /
  Last update: 2018-08-14 20:04    [W:0.310 / U:3.304 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site