LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Oleg Nesterov <oleg@tv-sign.ru>,
	mingo@elte.hu, xemul@openvz.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU
Date: Tue, 29 Jan 2008 20:56:33 -0800	[thread overview]
Message-ID: <20080130045633.GG12073@linux.vnet.ibm.com> (raw)
In-Reply-To: <m1r6g0gir1.fsf@ebiederm.dsl.xmission.com>

On Tue, Jan 29, 2008 at 07:16:50PM -0700, Eric W. Biederman wrote:
> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Tue, 29 Jan 2008 19:40:19 +0300
> > Oleg Nesterov <oleg@tv-sign.ru> wrote:
> >
> >> With CONFIG_PREEMPT_RCU read_lock(tasklist_lock) doesn't imply
> > rcu_read_lock(),
> >> but find_pid_ns()->hlist_for_each_entry_rcu() should be safe under tasklist.
> >> 
> >> Usually it is, detach_pid() is always called under write_lock(tasklist_lock),
> >> but copy_process() calls free_pid() lockless.
> >> 
> >> "#ifdef CONFIG_PREEMPT_RCU" is added mostly as documentation, perhaps it is
> >> too ugly and should be removed.
> >> 
> >> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
> >> 
> >> --- MM/kernel/fork.c~PR_RCU	2008-01-27 17:09:47.000000000 +0300
> >> +++ MM/kernel/fork.c	2008-01-29 19:23:44.000000000 +0300
> >> @@ -1335,8 +1335,19 @@ static struct task_struct *copy_process(
> >>  	return p;
> >>  
> >>  bad_fork_free_pid:
> >> -	if (pid != &init_struct_pid)
> >> +	if (pid != &init_struct_pid) {
> >> +#ifdef CONFIG_PREEMPT_RCU
> >> +		/*
> >> +		 * read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> >> +		 * make sure find_pid() is safe under read_lock(tasklist).
> >> +		 */
> >> +		write_lock_irq(&tasklist_lock);
> >> +#endif
> >>  		free_pid(pid);
> >> +#ifdef CONFIG_PREEMPT_RCU
> >> +		write_unlock_irq(&tasklist_lock);
> >> +#endif
> >> +	}
> >>  bad_fork_cleanup_namespaces:
> >>  	exit_task_namespaces(p);
> >>  bad_fork_cleanup_keys:
> >
> > My attempt to understand this change timed out.
> >
> > kernel/pid.c is full of global but undocumented functions.  What are the
> > locking requirements for free_pid()?  free_pid_ns()?  If it's just
> > caller-must-hold-rcu_read_lock() then why not use rcu_read_lock() here?
> >
> > If the locking is "caller must hold write_lock_irq(tasklist_lock) then the
> > sole relevant comment in there (in free_pid()) is wrong.
> >
> > Guys, more maintainable code please?
> 
> Well I took a quick look.
> 
> Yeah this looks complex.
> Mutation of the hash table is protected by pidmap_lock.
> But attachments of tasks to hash entries is protected task_lock.
> 
> And it looks like it has been that way since commit 92476d7fc0326a409ab1d3864a04093a6be9aca7
> 
> I thought free_pid did not have any requirements that a lock be held when
> it was called, taking all of the needed locks.
> 
> Now how read_lock doesn't imply rcu_read_lock is another question.

Although read_lock() does accidentally imply rcu_read_lock() for
Classic RCU, it no longer does so for preemptible RCU.

But I thought that we had found these -- must have missed some...

							Thanx, Paul

> Anyway I have to run.  I will see about looking at this in a bit.
> 
> Eric

  reply	other threads:[~2008-01-30  4:56 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-29 16:40 Oleg Nesterov
2008-01-29 23:02 ` Andrew Morton
2008-01-30 14:17   ` Peter Zijlstra
2008-01-31 13:32   ` Ingo Molnar
2008-01-29 23:08 ` Andrew Morton
2008-01-30  2:16   ` Eric W. Biederman
2008-01-30  4:56     ` Paul E. McKenney [this message]
2008-01-30  3:24 ` Eric W. Biederman
2008-01-30  5:00   ` Paul E. McKenney
2008-01-30  9:20     ` Eric W. Biederman
2008-01-30  9:48       ` Oleg Nesterov
2008-01-30  9:30   ` Oleg Nesterov
2008-01-30 18:28     ` Eric W. Biederman
2008-01-31  9:31       ` Oleg Nesterov
2009-12-14  2:15 Tetsuo Handa

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=20080130045633.GG12073@linux.vnet.ibm.com \
    --to=paulmck@linux.vnet.ibm.com \
    --cc=akpm@linux-foundation.org \
    --cc=ebiederm@xmission.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@elte.hu \
    --cc=oleg@tv-sign.ru \
    --cc=xemul@openvz.org \
    --subject='Re: [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU' \
    /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

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).