From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: Nicolai Stange <nicstange@gmail.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances
Date: Tue, 18 Apr 2017 08:17:00 -0700 [thread overview]
Message-ID: <20170418151700.GU3956@linux.vnet.ibm.com> (raw)
In-Reply-To: <1492522832.18845.1.camel@sipsolutions.net>
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
next prev parent reply other threads:[~2017-04-18 15:17 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-23 14:54 deadlock in synchronize_srcu() in debugfs? Johannes Berg
2017-03-23 15:29 ` Johannes Berg
2017-03-24 8:56 ` Johannes Berg
2017-03-24 9:24 ` Johannes Berg
2017-03-24 17:45 ` Paul E. McKenney
2017-03-24 18:51 ` Johannes Berg
2017-03-24 19:33 ` Paul E. McKenney
2017-03-24 20:20 ` Paul E. McKenney
2017-03-27 11:18 ` Johannes Berg
2017-03-23 15:36 ` Nicolai Stange
2017-03-23 15:47 ` Johannes Berg
2017-03-27 11:36 ` Johannes Berg
2017-03-30 7:32 ` Nicolai Stange
2017-03-30 7:55 ` Johannes Berg
2017-03-30 10:27 ` Nicolai Stange
2017-03-30 11:11 ` Johannes Berg
2017-03-31 9:03 ` Nicolai Stange
2017-03-31 9:44 ` Johannes Berg
2017-04-16 9:51 ` [RFC PATCH 0/9] debugfs: per-file removal protection Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 1/9] debugfs: add support for more elaborate ->d_fsdata Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 2/9] debugfs: implement per-file removal protection Nicolai Stange
2017-04-18 2:23 ` [lkp-robot] [debugfs] f3e7155d08: BUG:unable_to_handle_kernel kernel test robot
2017-04-23 18:37 ` Nicolai Stange
2017-04-24 6:36 ` Ye Xiaolong
2017-04-16 9:51 ` [RFC PATCH 3/9] debugfs: debugfs_real_fops(): drop __must_hold sparse annotation Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 4/9] debugfs: convert to debugfs_file_get() and -put() Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 5/9] IB/hfi1: " Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 6/9] debugfs: purge obsolete SRCU based removal protection Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 7/9] debugfs: call debugfs_real_fops() only after debugfs_file_get() Nicolai Stange
2017-04-16 9:51 ` [RFC PATCH 8/9] debugfs: defer debugfs_fsdata allocation to first usage Nicolai Stange
2017-04-18 9:36 ` Johannes Berg
2017-05-02 20:05 ` Nicolai Stange
2017-05-03 5:43 ` Johannes Berg
2017-04-16 9:51 ` [RFC PATCH 9/9] debugfs: free debugfs_fsdata instances Nicolai Stange
2017-04-17 16:01 ` Paul E. McKenney
2017-04-18 9:39 ` Johannes Berg
2017-04-18 13:31 ` Paul E. McKenney
2017-04-18 13:40 ` Johannes Berg
2017-04-18 15:17 ` Paul E. McKenney [this message]
2017-04-18 15:20 ` Johannes Berg
2017-04-18 17:19 ` Paul E. McKenney
2017-03-23 15:37 ` deadlock in synchronize_srcu() in debugfs? Paul E. McKenney
2017-03-23 15:46 ` Johannes Berg
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