Messages in this thread | | | Subject | Re: [GIT PULL rcu/urgent] yet more lockdep-RCU splat fixes | From | Peter Zijlstra <> | Date | Wed, 23 Jun 2010 10:08:47 +0200 |
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On Tue, 2010-06-22 at 13:44 -0700, Paul E. McKenney wrote:
> I am probably missing something, but I see wake_affine() only called > from select_task_rq_fair(), which is one of the possible values for > ->select_task_rq(). This can be called from select_task_rq(), which > claims that it can be called without holding rq->lock. I do not see > any rq->lock acquisition on the path from select_task_rq() to the > call to wake_affine(). > You're right, although try_to_wake_up(), wake_up_new_task() and sched_exec() all hold a rq->lock (not sufficient to cover both task_group() callers though).
I posted a patch yesterday that makes try_to_wake_up() call select_task_rq() without any rq->lock held (although its a scary patch and needs more work).
> rcu: apply RCU protection to wake_affine() > > The task_group() function returns a pointer that must be protected > by either RCU, the ->alloc_lock, or the cgroup lock (see the > rcu_dereference_check() in task_subsys_state(), which is invoked by > task_group()). The wake_affine() function currently does none of these, > which means that a concurrent update would be within its rights to free > the structure returned by task_group(). Because wake_affine() uses this > structure only to compute load-balancing heuristics, there is no reason > to acquire either of the two locks. > > Therefore, this commit introduces an RCU read-side critical section that > starts before the first call to task_group() and ends after the last use > of the "tg" pointer returned from task_group(). Thanks to Li Zefan for > pointing out the need to extend the RCU read-side critical section from > that proposed by the original patch. > > Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
OK, fair enough, thanks!
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