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Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances

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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.

johannes

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[RFC PATCH 0/9] debugfs: per-file removal protection Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 4/9] debugfs: convert to debugfs_file_get() and -put() Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 2/9] debugfs: implement per-file removal protection Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 5/9] IB/hfi1: convert to debugfs_file_get() and -put() Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 8/9] debugfs: defer debugfs_fsdata allocation to first usage Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
    Re: [RFC PATCH 8/9] debugfs: defer debugfs_fsdata allocation to  first usage Johannes Berg <johannes@sipsolutions.net> - 2017-04-18 11:40 +0200
  [RFC PATCH 6/9] debugfs: purge obsolete SRCU based removal protection Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
    Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-17 18:10 +0200
      Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances Johannes Berg <johannes@sipsolutions.net> - 2017-04-18 11:50 +0200
        Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 15:40 +0200
          Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances Johannes Berg <johannes@sipsolutions.net> - 2017-04-18 15:50 +0200
            Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 17:20 +0200
              Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances Johannes Berg <johannes@sipsolutions.net> - 2017-04-18 17:30 +0200
                Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 19:20 +0200
  [RFC PATCH 1/9] debugfs: add support for more elaborate ->d_fsdata Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 3/9] debugfs: debugfs_real_fops(): drop __must_hold sparse annotation Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200
  [RFC PATCH 7/9] debugfs: call debugfs_real_fops() only after debugfs_file_get() Nicolai Stange <nicstange@gmail.com> - 2017-04-16 12:00 +0200

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