Linux-rt-users archive on
 help / color / Atom feed
From: Steven Rostedt <>
To: Sebastian Andrzej Siewior <>
Cc: LKML <>,
	linux-rt-users <>,
	Thomas Gleixner <>,
	"Paul E. McKenney" <>
Subject: Re: [PATCH RT] locking: Make spinlock_t and rwlock_t a RCU section on RT
Date: Mon, 25 Nov 2019 12:25:45 -0500
Message-ID: <20191125122545.20e721d7@gandalf.local.home> (raw)
In-Reply-To: <>

On Fri, 22 Nov 2019 19:01:40 +0100
Sebastian Andrzej Siewior <> wrote:

> Let me give you an example how I got into this:
> do_sigaction() acquires p->sighand->siglock and then iterates over list
> via for_each_thread() which is a list_for_each_entry_rcu(). No RCU lock
> is held, just the siglock.
> On removal side, __unhash_process() removes a task from the list but
> while doing so it holds the siglock and tasklist_lock. So it is
> perfectly fine.
> Later, we have:
> |do_exit()
> | -> exit_notify()
> |   -> write_lock_irq(&tasklist_lock);
> |   -> forget_original_parent()
> |      -> find_child_reaper()
> |        -> find_alive_thread()
> |           -> for_each_thread()
> find_alive_thread() does the for_each_thread() and checks PF_EXITING.
> it might be enough for not operating on "removed" task_struct. It
> dereferences task_struct->flags while looking for PF_EXITING. At this
> point only tasklist_lock is acquired.
> I have *no* idea if the whole synchronisation based on siglock/
> PF_EXITING/ tasklist_lock is enough and RCU simply doesn't matter. It
> seems so.
> I am a little worried if this construct here (or somewhere else) assumes
> that holding one of those locks, which disable preemption, is the same
> as rcu_read_lock() (or rcu_read_lock_sched()).

I'm wondering if instead, we should start throwing in rcu_read_lock()
and explicitly have the preempt disabled rcu use that as well, since
today it's basically one and the same.

Although, we still need to be careful for some special cases like
function tracing, that uses its own kind of RCU. But that should be
fixed when I introduce rcu_read_lock_tasks() :-)

-- Steve

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-19  8:46 Sebastian Andrzej Siewior
2019-11-19 14:21 ` Steven Rostedt
2019-11-19 14:46   ` Paul E. McKenney
2019-11-22 18:01   ` Sebastian Andrzej Siewior
2019-11-25 17:25     ` Steven Rostedt [this message]
2019-11-29 15:45       ` Sebastian Andrzej Siewior
2019-11-29 22:51         ` Paul E. McKenney
2019-11-19 14:47 ` Paul E. McKenney

Reply instructions:

You may reply publically 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:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20191125122545.20e721d7@gandalf.local.home \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-rt-users archive on

Archives are clonable:
	git clone --mirror linux-rt-users/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-rt-users linux-rt-users/ \
	public-inbox-index linux-rt-users

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone