Messages in this thread | | | Date | Tue, 18 Apr 2017 08:17:00 -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 03:40:32PM +0200, Johannes Berg wrote: > On Tue, 2017-04-18 at 06:31 -0700, Paul E. McKenney wrote: > > 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(); > > Ok, that's not something that happens here either. > > > 2. rcu_read_lock(); > > schedule_timeout_interruptible(HZ); > > rcu_read_unlock(); > > Neither is this happening. > > > There are more, but this should get you the flavor of the types > > of bugs CONFIG_PROVE_RCU=y can locate for you. > > Makes sense. However, the issue at hand is what we (you and I) > discussed earlier wrt. lockdep -- from SRCU's point of view everything > is actually OK, except that the one thread is waiting for something and > we can never finish the grace period, and thus synchronize_srcu() will > never return. But there's no real SRCU bug here. > > > > 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. > > There's no SRCU deadlock though. I know exactly why it happens, in my > case, which is the following: > > Thread 1 > userspace: read(debugfs_file_1) > srcu_read_lock(&debugfs_srcu); // in debugfs bowels > wait_event_interruptible(...); // in my driver's debugfs read method > > Thread 2: > debugfs_remove(debugfs_file_2); > srcu_synchronize(&debugfs_srcu); // in debugfs bowels > > > This is the live-lock. The deadlock is something I posited but never > ran into: > > CPU 1 CPU 2 > srcu_read_lock(&debugfs_srcu); > rtnl_lock(); > rtnl_lock(); > srcu_synchronize(&debugfs_srcu); > > Again, no (S)RCU abuse here, just an ABBA deadlock.
OK, please accept my apologies for failing to follow the thread.
I nevertheless reiterate my advice to run at least some tests with CONFIG_PROVE_RCU=y. And yes, it would be good to upgrade lockdep to find the above theoretical deadlock.
Thanx, Paul
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