Messages in this thread | | | Date | Wed, 16 Jun 2010 15:41:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [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
| |