Messages in this thread | | | Date | Thu, 6 Aug 2020 06:35:57 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Finally starting on short RCU grace periods, but... |
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On Thu, Aug 06, 2020 at 03:25:57PM +0200, Dmitry Vyukov wrote: > On Thu, Aug 6, 2020 at 3:22 PM Dmitry Vyukov <dvyukov@google.com> wrote: > > > > On Thu, Aug 6, 2020 at 12:31 PM Marco Elver <elver@google.com> wrote: > > > > > > +Cc kasan-dev
Thank you!
> > > On Thu, 6 Aug 2020 at 01:08, Paul E. McKenney <paulmck@kernel.org> wrote: > > > > > > > > Hello! > > > > > > > > If I remember correctly, one of you asked for a way to shorten RCU > > > > grace periods so that KASAN would have a better chance of detecting bugs > > > > such as pointers being leaked out of RCU read-side critical sections. > > > > I am finally starting entering and testing code for this, but realized > > > > that I had forgotten a couple of things: > > > > > > > > 1. I don't remember exactly who asked, but I suspect that it was > > > > Kostya. I am using his Reported-by as a placeholder for the > > > > moment, but please let me know if this should be adjusted. > > > > > > It certainly was not me. > > > > > > > 2. Although this work is necessary to detect situtions where > > > > call_rcu() is used to initiate a grace period, there already > > > > exists a way to make short grace periods that are initiated by > > > > synchronize_rcu(), namely, the rcupdate.rcu_expedited kernel > > > > boot parameter. This will cause all calls to synchronize_rcu() > > > > to act like synchronize_rcu_expedited(), resulting in about 2-3 > > > > orders of magnitude reduction in grace-period latency on small > > > > systems (say 16 CPUs). > > > > > > > > In addition, I plan to make a few other adjustments that will > > > > increase the probability of KASAN spotting a pointer leak even in the > > > > rcupdate.rcu_expedited case. > > > > > > Thank you, that'll be useful I think. > > > > > > > But if you would like to start this sort of testing on current mainline, > > > > rcupdate.rcu_expedited is your friend! > > > > Hi Paul, > > > > This is great! > > > > I understand it's not a sufficiently challenging way of tracking > > things, but it's simply here ;) > > https://bugzilla.kernel.org/show_bug.cgi?id=208299 > > (now we also know who asked for this, +Jann)
Thank you, and I will update the Reported-by lines accordingly.
> > I've tested on the latest mainline and with rcupdate.rcu_expedited=1 > > it boots to ssh successfully and I see: > > [ 0.369258][ T0] All grace periods are expedited (rcu_expedited). > > > > I have created https://github.com/google/syzkaller/pull/2021 to enable > > it on syzbot. > > On syzbot we generally use only 2-4 CPUs per VM, so it should be even better.
Sounds good, and perhaps this will answer Marco's question below. ;-)
> > > Do any of you remember some bugs we missed due to this? Can we find > > > them if we add this option? > > > > The problem is that it's hard to remember bugs that were not caught :) > > Here is an approximation of UAFs with free in rcu callback: > > https://groups.google.com/forum/#!searchin/syzkaller-bugs/KASAN$20use-after-free$20rcu_do_batch%7Csort:date > > The ones with low hit count are the ones that we almost did not catch. > > That's the best estimation I can think of. Also potentially we can get > > reproducers for such bugs without reproducers. > > Maybe we will be able to correlate some bugs/reproducers that appear > > soon with this change. > > Wait, it was added in 2012? > https://github.com/torvalds/linux/commit/3705b88db0d7cc4
Indeed it was, which is my current excuse for having failed to immediately mention it to Jann during our IRC discussion.
The purpose back then was to make battery-powered systems go faster, I think mostly focused on CPU hotplug operations. At least that would explain the commit log being indefinite on the exact benefit. ;-)
Thanx, Paul
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