All of lore.kernel.org
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>,
	Peter Zijlstra <peterz@infradead.org>,
	Chris Metcalf <cmetcalf@ezchip.com>,
	Christoph Lameter <cl@linux.com>, Kirill Tkhai <tkhai@yandex.ru>,
	Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value
Date: Fri, 30 Aug 2019 09:58:15 -0700	[thread overview]
Message-ID: <CAHk-=whtj+aSYftniMRG4xvFE8dmmYyrqcJyPmzStsfj5w9r=w@mail.gmail.com> (raw)
In-Reply-To: <20190830164436.GD2634@redhat.com>

On Fri, Aug 30, 2019 at 9:44 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> ->curr is not protected by RCU, the last schedule does put_task_struct()
> in finish_task_switch().

Right you are.

It's only the sighand allocation itself that is RCU-allocated (using
SLAB_TYPESAFE_BY_RCU, so only the backing page freeing is RCU-delayed,
it can be re-used immediately).

For some reason I thought the main thread struct was too, but that was
just my fevered imagination.

> Of course we can change this and add another call_rcu (actually we can do
> better), and after that we do not need task_rcu_dereference() at all.

No, we wouldn't do another RCU call, we'd just make task_struct_cachep
be SLAB_TYPESAFE_BY_RCU too. In the reuse case, you have no cost at
all.

However, the overhead of RCU freeing is real. It's much less for the
SLAB_TYPESAFE_BY_RCU case (at least for small allocations) than for
"free every single allocaiton by RCU", but it's still very real.

(For small allocations, you only take the RCU hit when you free the
backing store of the pages, which is much less frequent - but for
something big like the task_struct, I don't know how much of a
buffering the slab allocation ends up being)

So it's probably better to take the hit at task_rcu_dereference() than
add RCU logic to the task freeing.

              Linus

  reply	other threads:[~2019-08-30 16:58 UTC|newest]

Thread overview: 75+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-30 14:08 [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value Russell King - ARM Linux admin
2019-08-30 15:24 ` Oleg Nesterov
2019-08-30 15:30 ` Linus Torvalds
2019-08-30 15:40   ` Russell King - ARM Linux admin
2019-08-30 15:43     ` Linus Torvalds
2019-08-30 15:41   ` Linus Torvalds
2019-08-30 16:09     ` Oleg Nesterov
2019-08-30 16:21       ` Linus Torvalds
2019-08-30 16:44         ` Oleg Nesterov
2019-08-30 16:58           ` Linus Torvalds [this message]
2019-08-30 19:36         ` Eric W. Biederman
2019-09-02 13:40           ` Oleg Nesterov
2019-09-02 13:53             ` Peter Zijlstra
2019-09-02 14:44               ` Oleg Nesterov
2019-09-02 16:20                 ` Peter Zijlstra
2019-09-02 17:04             ` Eric W. Biederman
2019-09-02 17:34               ` Linus Torvalds
2019-09-03  4:50                 ` [PATCH 0/3] task: Making tasks on the runqueue rcu protected Eric W. Biederman
2019-09-03  4:51                   ` [PATCH 1/3] task: Add a count of task rcu users Eric W. Biederman
2019-09-04 14:36                     ` Oleg Nesterov
2019-09-04 14:44                     ` Frederic Weisbecker
2019-09-04 15:32                       ` Oleg Nesterov
2019-09-04 16:33                         ` Frederic Weisbecker
2019-09-04 18:20                           ` Linus Torvalds
2019-09-05 14:59                             ` Frederic Weisbecker
2019-09-03  4:52                   ` [PATCH 2/3] task: RCU protect tasks on the runqueue Eric W. Biederman
2019-09-03  7:41                     ` Peter Zijlstra
2019-09-03  7:47                       ` Peter Zijlstra
2019-09-03 16:44                         ` Eric W. Biederman
2019-09-03 17:08                           ` Linus Torvalds
2019-09-03 18:13                             ` Eric W. Biederman
2019-09-03 19:18                               ` Linus Torvalds
2019-09-03 20:06                                 ` Peter Zijlstra
2019-09-03 21:32                                   ` Paul E. McKenney
2019-09-05 20:02                                     ` Eric W. Biederman
2019-09-05 20:55                                       ` Paul E. McKenney
2019-09-06  7:07                                       ` Peter Zijlstra
2019-09-09 12:22                                         ` Eric W. Biederman
2019-09-25  7:36                                           ` Peter Zijlstra
2019-09-27  8:10                                   ` [tip: sched/urgent] tasks, sched/core: RCUify the assignment of rq->curr tip-bot2 for Eric W. Biederman
2019-09-03 19:42                               ` [PATCH 2/3] task: RCU protect tasks on the runqueue Peter Zijlstra
2019-09-14 12:31                           ` [PATCH v2 1/4] task: Add a count of task rcu users Eric W. Biederman
2019-09-14 12:31                           ` [PATCH v2 2/4] task: Ensure tasks are available for a grace period after leaving the runqueue Eric W. Biederman
2019-09-14 12:32                           ` [PATCH v2 3/4] task: With a grace period after finish_task_switch, remove unnecessary code Eric W. Biederman
2019-09-04 14:22                     ` [PATCH 2/3] task: RCU protect tasks on the runqueue Frederic Weisbecker
2019-09-03  4:52                   ` [PATCH 3/3] task: Clean house now that tasks on the runqueue are rcu protected Eric W. Biederman
2019-09-03  9:45                     ` kbuild test robot
2019-09-03 13:06                     ` Oleg Nesterov
2019-09-03 13:58                   ` [PATCH 0/3] task: Making tasks on the runqueue " Oleg Nesterov
2019-09-03 15:44                   ` Linus Torvalds
2019-09-03 19:46                     ` Peter Zijlstra
     [not found]                   ` <87muf7f4bf.fsf_-_@x220.int.ebiederm.org>
2019-09-14 12:33                     ` [PATCH v2 1/4] task: Add a count of task rcu users Eric W. Biederman
2019-09-15 13:54                       ` Paul E. McKenney
2019-09-27  8:10                       ` [tip: sched/urgent] tasks: Add a count of task RCU users tip-bot2 for Eric W. Biederman
2019-09-14 12:33                     ` [PATCH v2 2/4] task: Ensure tasks are available for a grace period after leaving the runqueue Eric W. Biederman
2019-09-15 14:07                       ` Paul E. McKenney
2019-09-15 14:09                         ` Paul E. McKenney
2019-09-27  8:10                       ` [tip: sched/urgent] tasks, sched/core: " tip-bot2 for Eric W. Biederman
2019-09-14 12:34                     ` [PATCH v2 3/4] task: With a grace period after finish_task_switch, remove unnecessary code Eric W. Biederman
2019-09-15 14:32                       ` Paul E. McKenney
2019-09-15 17:07                         ` Linus Torvalds
2019-09-15 18:47                           ` Paul E. McKenney
2019-09-27  8:10                       ` [tip: sched/urgent] tasks, sched/core: With a grace period after finish_task_switch(), " tip-bot2 for Eric W. Biederman
2019-09-14 12:35                     ` [PATCH v2 4/4] task: RCUify the assignment of rq->curr Eric W. Biederman
2019-09-15 14:41                       ` Paul E. McKenney
2019-09-15 17:59                         ` Eric W. Biederman
2019-09-15 18:25                           ` Eric W. Biederman
2019-09-15 18:48                             ` Paul E. McKenney
2019-09-20 23:02                       ` Frederic Weisbecker
2019-09-26  1:49                         ` Eric W. Biederman
2019-09-26 12:42                           ` Frederic Weisbecker
2019-09-14 17:43                     ` [PATCH v2 0/4] task: Making tasks on the runqueue rcu protected Linus Torvalds
2019-09-17 17:38                       ` Eric W. Biederman
2019-09-25  7:51                         ` Peter Zijlstra
2019-09-26  1:11                           ` Eric W. Biederman

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='CAHk-=whtj+aSYftniMRG4xvFE8dmmYyrqcJyPmzStsfj5w9r=w@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=cl@linux.com \
    --cc=cmetcalf@ezchip.com \
    --cc=efault@gmx.de \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux@armlinux.org.uk \
    --cc=mingo@kernel.org \
    --cc=oleg@redhat.com \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=tkhai@yandex.ru \
    /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 an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.