From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Mike Galbraith <efault@gmx.de>
Cc: Paul Menage <menage@google.com>, Li Zefan <lizf@cn.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>,
Colin Cross <ccross@android.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Ingo Molnar <mingo@elte.hu>
Subject: Re: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task
Date: Mon, 2 May 2011 06:46:35 -0700 [thread overview]
Message-ID: <20110502134635.GB4197@linux.vnet.ibm.com> (raw)
In-Reply-To: <1304080487.21536.16.camel@marge.simson.net>
On Fri, Apr 29, 2011 at 02:34:47PM +0200, Mike Galbraith wrote:
> (ok crickets, keep the noise down please)
>
> On Thu, 2011-04-28 at 11:38 +0200, Mike Galbraith wrote:
>
> > The explosions are because the logic to snag rmdir() should anyone grab
> > a reference will let us zip right through and free a cgroup while
> > there's a destruction in flight. Adding a cgrp->count check before
> > trying to cgroup_clear_css_refs() prevents the explosions, but that
> > leaves RCU grace periods still embedded in userspace.
> >
> > So...
> >
> > I bent up put_css_set() a bit to try to destroy synchronously on final
> > put if possible, so rmdir() will only be snagged if that fails. The
> > thing seems to be working, so I'll show it. Readers (beware) may notice
> > some gratuitous looking chicken scratches. Just ignore those, and move
> > along smartly to the suggesting a much better way part, for surely one
> > must exist.
>
> Hi Self, (*howdy*)
>
> You might try the below. No weird gyrations to accomplish the same
> thing, and I see no slub debug gripes, no list debug gripes, nada.
>
> Makes one wonder what these things do for a living.
If you are adding something to an RCU-protected data structure, then you do
not need synchronize_rcu(). But if you are removing something from
an RCU-protected data structure, then you really do need them. If you
leave them out, you can see the following type of failure:
1. CPU 0, running in an RCU read-side critical section, obtains
a pointer to data item A.
2. CPU 1 removes data item A from the structure.
3. CPU 1 does not do a synchronize_rcu(). If CPU 1 had done a
synchronize_rcu(), then it would have waited until CPU 0 had
left its RCU read-side critical section, and thus until
CPU 0 stopped using its pointer to data item A. But there was
no synchronize_rcu(), so CPU 0 is still looking at data item A.
4. CPU 1 frees data item A.
This is very bad. CPU 0 has a pointer into the freelist. Worse yet,
some other CPU might allocate memory and get a pointer to data item A.
That CPU and CPU 0 would then have an interesting but short lived
disagreement about that memory's type. Crash goes the kernel.
So please do not remove synchronize_rcu() calls unless you can prove
that it is safe to do so!
Thanx, Paul
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 25c7eb5..b8c64bf 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -826,13 +826,6 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
> struct cgroup *cgrp = dentry->d_fsdata;
> struct cgroup_subsys *ss;
> BUG_ON(!(cgroup_is_removed(cgrp)));
> - /* It's possible for external users to be holding css
> - * reference counts on a cgroup; css_put() needs to
> - * be able to access the cgroup after decrementing
> - * the reference count in order to know if it needs to
> - * queue the cgroup to be handled by the release
> - * agent */
> - synchronize_rcu();
>
> mutex_lock(&cgroup_mutex);
> /*
> @@ -1822,7 +1815,6 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
> ss->attach(ss, cgrp, oldcgrp, tsk, false);
> }
> set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
> - synchronize_rcu();
> put_css_set(cg);
>
> /*
>
>
> --
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next prev parent reply other threads:[~2011-05-02 13:46 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-07 9:55 query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task Mike Galbraith
2011-04-13 2:02 ` Li Zefan
2011-04-13 3:11 ` Mike Galbraith
2011-04-13 13:16 ` Paul Menage
2011-04-13 16:56 ` Mike Galbraith
2011-04-14 7:26 ` Mike Galbraith
2011-04-14 8:34 ` Mike Galbraith
2011-04-14 8:44 ` Mike Galbraith
2011-04-18 14:21 ` Mike Galbraith
2011-04-28 9:38 ` Mike Galbraith
2011-04-29 12:34 ` Mike Galbraith
2011-05-02 13:46 ` Paul E. McKenney [this message]
2011-05-02 14:29 ` Mike Galbraith
2011-05-02 15:04 ` Mike Galbraith
2011-05-02 23:03 ` Paul E. McKenney
2011-04-13 13:10 ` Paul Menage
2011-04-13 16:52 ` Mike Galbraith
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