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 06:31:36 -0700 [thread overview]
Message-ID: <20170418133136.GS3956@linux.vnet.ibm.com> (raw)
In-Reply-To: <1492508367.2472.9.camel@sipsolutions.net>
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
next prev parent reply other threads:[~2017-04-18 13:31 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 [this message]
2017-04-18 13:40 ` Johannes Berg
2017-04-18 15:17 ` Paul E. McKenney
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170418133136.GS3956@linux.vnet.ibm.com \
--to=paulmck@linux.vnet.ibm.com \
--cc=gregkh@linuxfoundation.org \
--cc=johannes@sipsolutions.net \
--cc=linux-kernel@vger.kernel.org \
--cc=nicstange@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).