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stop_machine: Disable preemption after queueing stopper threads
This commit:9fb8d5dc4b
("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:9fb8d5dc4b
("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>
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@ -260,6 +260,15 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
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err = 0;
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__cpu_stop_queue_work(stopper1, work1, &wakeq);
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__cpu_stop_queue_work(stopper2, work2, &wakeq);
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/*
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* The waking up of stopper threads has to happen
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* in the same scheduling context as the queueing.
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* Otherwise, there is a possibility of one of the
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* above stoppers being woken up by another CPU,
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* and preempting us. This will cause us to n ot
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* wake up the other stopper forever.
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*/
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preempt_disable();
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unlock:
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raw_spin_unlock(&stopper2->lock);
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raw_spin_unlock_irq(&stopper1->lock);
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@ -271,7 +280,6 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
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}
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if (!err) {
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preempt_disable();
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wake_up_q(&wakeq);
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preempt_enable();
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}
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