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SubjectRe: [GIT PULL rcu/urgent] yet more lockdep-RCU splat fixes
On Wed, Jun 16, 2010 at 08:23:51AM +0200, Peter Zijlstra wrote:
> On Wed, 2010-06-16 at 07:53 +0200, Ingo Molnar wrote:
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > Hello, Ingo,
> > >
> > > Here are a few more fixes for RCU-lockdep splats.
> > >
> > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent
> > >
> > > This is based off of v2.6.35-rc3.
> > >
> > > Thanx, Paul
> > >
> > > ------------------>
> > > Daniel J Blueman (1):
> > > rcu: fix lockdep splat in wake_affine()
> > >
> > > Paul E. McKenney (2):
> > > rcu: fix scope of wake_affine()'s new RCU read-side critical section
> > > idr: fix RCU lockdep splat in idr_get_next()
> > >
> > > kernel/sched_fair.c | 2 ++
> > > lib/idr.c | 4 ++--
> > > 2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > Pulled, thanks a lot Paul!
>
> I'm not at all sure of those two wake_affine() ones..

Hello, Peter!

Here is the story as I understand it:

o wake_affine() calls task_group() and uses the resulting
pointer, for example, passing it to effective_load().

This pointer is to a struct task_group, which contains
a struct rcu_head, which is passed to call_rcu in
sched_destroy_group(). So some protection really is
needed -- or is it enough that wake_affine seems to be
invoked on the current task? If the latter, we would
need to add a "task == current" check to task_subsys_state().

o task_group() calls task_subsys_state(), returning a pointer to
the enclosing task_group structure.

o task_subsys_state() returns an rcu_dereference_check()ed
pointer. The caller must either be in an RCU read-side
critical section, hold the ->alloc_lock, or hold the
cgroup lock.

Now wake_affine() appears to be doing load calculations, so it does not
seem reasonable to acquire the lock. Hence the use of RCU.

So, what should we be doing instead? ;-)

Thanx, Paul


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