On Thu, Jun 12, 2014 at 11:40:07PM +0200, Thomas Gleixner wrote: > On Thu, 12 Jun 2014, Paul E. McKenney wrote: > > On Thu, Jun 12, 2014 at 07:28:44PM +0200, Oleg Nesterov wrote: > > > On 06/11, Paul E. McKenney wrote: > > > > > > > > On Wed, Jun 11, 2014 at 07:59:34PM +0200, Oleg Nesterov wrote: > > > > > On 06/11, Paul E. McKenney wrote: > > > > > > > > > > > > I was thinking of ->boost_completion as the way to solve it easily, > > > > > > but > > > > > > what did you have in mind? > > > > > > > > > > I meant, rcu_boost() could probably just do "mtx->owner = t", we know > > > > > that > > > > > it was unlocked by us and nobody else can use it until we set > > > > > t->rcu_boost_mutex. > > > > > > > > My concern with this is that rcu_read_unlock_special() could > > > > hypothetically > > > > get preempted (either by kernel or hypervisor), so that it might be a > > > > long > > > > time until it makes its reference. But maybe that reference would be > > > > harmless in this case. > > > > > > Confused... Not sure I understand what did you mean, and certainly I do > > > not > > > understand how this connects to the proxy-locking method. > > > > > > Could you explain? > > > > Here is the hypothetical sequence of events, which cannot happen unless > > the CPU releasing the lock accesses the structure after releasing > > the lock: > > > > CPU 0 CPU 1 (booster) > > > > releases boost_mutex > > > > acquires boost_mutex > > releases boost_mutex > > post-release boost_mutex access? > > Loops to next task to boost > > proxy-locks boost_mutex > > > > post-release boost_mutex access: > > confused due to proxy-lock > > operation? > > > > Now maybe this ends up being safe, but it sure feels like an accident > > waiting to happen. Some bright developer comes up with a super-fast > > handoff, and blam, RCU priority boosting takes it in the shorts. ;-) > > In contrast, using the completion prevents this. > > > > > > > And if we move it into rcu_node, then we can probably kill > > > > > ->rcu_boost_mutex, > > > > > rcu_read_unlock_special() could check rnp->boost_mutex->owner == > > > > > current. > > > > > > > > If this was anywhere near a hot code path, I would be sorely tempted. > > > > > > Ah, but I didn't mean perfomance. I think it is always good to try to > > > remove > > > something from task_struct, it is huge. I do not mean sizeof() in the > > > first > > > place, the very fact that I can hardly understand the purpose of a half > > > of its > > > members makes me sad ;) > > > > Now -that- just might make a huge amount of sense! Let's see... > > > > o We hold the rcu_node structure's ->lock when checking the owner > > (looks like rt_mutex_owner() is the right API). > > > > o We hold the rcu_node structure's ->lock when doing > > rt_mutex_init_proxy_locked(). > > > > o We -don't- hold ->lock when releasing the rt_mutex, but that > > should be OK: The owner is releasing it, and it is going to > > not-owned, so no other task can possibly see ownership moving > > to/from them. > > > > o The rcu_node structure grows a bit, but not enough to worry > > about, and on most systems, the decrease in task_struct size > > will more than outweigh the increase in rcu_node size. > > > > Looks quite promising, how about the following? (Hey, it builds, so it > > must be correct, right?) > > True. Why should we have users if we would test the crap we produce?
Well, it seems to be passing initial tests as well. Must be my tests need more work. > Just FYI, I have a patch pending which makes the release safe. > > http://marc.info/?l=linux-kernel&m=140251240630730&w=2 Very good, belt -and- suspenders! Might even work that way. ;-) I could argue for a cpu_relax() in the "while (!rt_mutex_has_waiters(lock))" loop for the case in which the CPU enqueuing itself is preempted, possibly by a hypervisor, but either way: Acked-by: Paul E. McKenney <[email protected]> Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
