From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
"Theodore Y. Ts'o" <tytso@mit.edu>,
Ext4 Developers List <linux-ext4@vger.kernel.org>,
Suraj Jitindar Singh <surajjs@amazon.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations
Date: Tue, 25 Feb 2020 08:38:27 -0800 [thread overview]
Message-ID: <20200225163826.GW2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <CAEXW_YQbiHW=yKiYAs0=8Mp84W6UunM6OOkHE66yXT6cSchm7A@mail.gmail.com>
On Tue, Feb 25, 2020 at 09:17:11AM -0500, Joel Fernandes wrote:
> On Mon, Feb 24, 2020 at 10:55 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> [...]
> > > > As for "task_struct's rcu_read_lock_nesting". Will it be enough just
> > > > have a look at preempt_count of current process? If we have for example
> > > > nested rcu_read_locks:
> > > >
> > > > <snip>
> > > > rcu_read_lock()
> > > > rcu_read_lock()
> > > > rcu_read_lock()
> > > > <snip>
> > > >
> > > > the counter would be 3.
> > >
> > > No, because preempt_count is not incremented during rcu_read_lock(). RCU
> > > reader sections can be preempted, they just cannot goto sleep in a reader
> > > section (unless the kernel is RT).
> >
> > You are both right.
> >
> > Vlad is correct for CONFIG_PREEMPT=n and CONFIG_DEBUG_ATOMIC_SLEEP=y
> > and Joel is correct otherwise, give or take the possibility of other
> > late-breaking corner cases. ;-)
>
> Oh yes, but even for PREEMPT=n, rcu_read_lock() is just a NOOP for
> that configuration and doesn't really mess around with preempt_count
> if I recall :-D. (doesn't need to mess with preempt_count because
> being in kernel mode is non-preemptible for PREEMPT=n anyway).
For PREEMPT=n, rcu_read_lock() is preempt_disable(), see the code
in include/linux/rcupdate.h. ;-)
Thanx, Paul
next prev parent reply other threads:[~2020-02-25 16:42 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-15 23:38 [PATCH RFC] ext4: fix potential race between online resizing and write operations Theodore Y. Ts'o
2020-02-16 12:12 ` Paul E. McKenney
2020-02-16 20:32 ` Theodore Y. Ts'o
2020-02-17 16:08 ` Uladzislau Rezki
2020-02-17 19:33 ` Theodore Y. Ts'o
2020-02-18 17:08 ` Uladzislau Rezki
2020-02-20 4:52 ` Theodore Y. Ts'o
2020-02-21 0:30 ` Paul E. McKenney
2020-02-21 13:14 ` Uladzislau Rezki
2020-02-21 20:22 ` Paul E. McKenney
2020-02-22 22:24 ` Joel Fernandes
2020-02-23 1:10 ` Paul E. McKenney
2020-02-24 17:40 ` Uladzislau Rezki
2020-02-25 2:07 ` Joel Fernandes
2020-02-25 3:55 ` Paul E. McKenney
2020-02-25 14:17 ` Joel Fernandes
2020-02-25 16:38 ` Paul E. McKenney [this message]
2020-02-25 17:00 ` Joel Fernandes
2020-02-25 18:54 ` Uladzislau Rezki
2020-02-25 22:47 ` Paul E. McKenney
2020-02-26 13:04 ` Uladzislau Rezki
2020-02-26 15:06 ` Paul E. McKenney
2020-02-26 15:53 ` Uladzislau Rezki
2020-02-27 14:08 ` Joel Fernandes
2020-03-01 11:13 ` Uladzislau Rezki
2020-02-27 13:37 ` Joel Fernandes
2020-03-01 11:08 ` Uladzislau Rezki
2020-03-01 12:07 ` Uladzislau Rezki
2020-02-25 2:11 ` Joel Fernandes
2020-02-21 12:06 ` Joel Fernandes
2020-02-21 13:28 ` Joel Fernandes
2020-02-21 19:21 ` Uladzislau Rezki
2020-02-21 19:25 ` Uladzislau Rezki
2020-02-22 22:12 ` Joel Fernandes
2020-02-24 17:02 ` Uladzislau Rezki
2020-02-24 23:14 ` Paul E. McKenney
2020-02-25 1:48 ` Joel Fernandes
2020-02-19 3:09 ` Jitindar SIngh, Suraj
2020-02-20 4:34 ` Theodore Y. Ts'o
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