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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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[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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