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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

  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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