Re: Lockdep splat from destroy_workqueue() with RT_PREEMPT_FULL
From: John Keeping <hidden>
Date: 2016-12-08 16:25:57
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Hi Sebastian, On Thu, 8 Dec 2016 14:33:06 +0100, Sebastian Andrzej Siewior wrote:
On 2016-12-08 12:20:28 [+0000], John Keeping wrote:quoted
I am seeing the following splat when stopping btattach on v4.4.30-rt41 with PREEMPT_RT_FULL with lockdep and slub_debug. The bad unlock balance seems to just be an effect of the lock having been overwritten with POISON_FREE, the real issue is that put_pwq_unlocked() is not resuming and unlocking the pool before the RCU work scheduled indirectly by put_pwq() has completed.can you reproduce this? If so, is this patch helping?
Yes, I am able to reproduce it reasonably reliably, or at least I *was* able to: since applying the patch below I haven't seen it fail at all, so consider the patch: Tested-by: John Keeping <redacted>
quoted hunk ↗ jump to hunk
diff --git a/kernel/workqueue.c b/kernel/workqueue.c --- a/kernel/workqueue.c +++ b/kernel/workqueue.c@@ -1135,9 +1135,11 @@ static void put_pwq_unlocked(struct pool_workqueue *pwq) * As both pwqs and pools are RCU protected, the * following lock operations are safe. */ + rcu_read_lock(); local_spin_lock_irq(pendingb_lock, &pwq->pool->lock); put_pwq(pwq); local_spin_unlock_irq(pendingb_lock, &pwq->pool->lock); + rcu_read_unlock(); } }
John