Messages in this thread | | | Date | Tue, 18 Apr 2017 06:31:36 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances |
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On Tue, Apr 18, 2017 at 11:39:27AM +0200, Johannes Berg wrote: > On Mon, 2017-04-17 at 09:01 -0700, Paul E. McKenney wrote: > > > If you have not already done so, please run this with debug enabled, > > especially CONFIG_PROVE_LOCKING=y (which implies CONFIG_PROVE_RCU=y). > > This is important because there are configurations for which the > > deadlocks you saw with SRCU turn into silent failure, including > > memory corruption. > > CONFIG_PROVE_RCU=y will catch many of those situations. > > Can you elaborate on that? I think we may have had CONFIG_PROVE_RCU > enabled in the builds where we saw the problem, but I'm not sure.
CONFIG_PROVE_RCU=y will reliably catch things like this:
1. rcu_read_lock(); synchronize_rcu(); rcu_read_unlock();
With CONFIG_PROVE_RCU=n and CONFIG_PREEMPT=n, this will result in too-short grace periods, which can free things out from under the read-side critical section, which in turn can result in arbitrary memory corruption. You might not even get a "scheduling while atomic", though CONFIG_PREEMPT_COUNT=y will produce this message.
With CONFIG_PREEMPT=y, on the other hand, this should deadlock in a manner similar to the earlier SRCU deadlocks seen in debugfs.
2. rcu_read_lock(); schedule_timeout_interruptible(HZ); rcu_read_unlock();
With CONFIG_PROVE_RCU=y and CONFIG_PREEMPT=y, this will just work, more or less. Until someone runs with CONFIG_PREEMPT=n, which will produce "scheduling while atomic". (I have a fix for this queued for 4.13, FWIW, so that in the future CONFIG_PROVE_RCU=y and CONFIG_PREEMPT=y will complain about this. But for now, silent bug.)
There are more, but this should get you the flavor of the types of bugs CONFIG_PROVE_RCU=y can locate for you.
> Can you say which configurations you're thinking of? And perhaps what > kind of corruption you're thinking of also? I'm having a hard time > imagining any corruption that should happen?
#1 is the silent corruption case given CONFIG_PROVE_RCU=n, CONFIG_PREEMPT=n, and CONFIG_PREEMPT_COUNT=n.
> Nicolai probably never even ran into this problem, though it should be > easy to reproduce.
I am just worried that the situation resulting in the earlier SRCU deadlocks might be hiding behind CONFIG_PROVE_RCU=n, CONFIG_PREEMPT=n, and CONFIG_PREEMPT_COUNT=n. Or some other bug hiding behind some other set of Kconfig options.
Thanx, Paul
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