Messages in this thread | | | Date | Thu, 6 Aug 2020 13:58:57 -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 06:35:57AM -0700, Paul E. McKenney wrote: > 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. ;-)
And don't look now, but my current intermediate state seems to make the following splat happen semi-reliably from within rcutorture when running scenario TREE01 with "--kconfig "CONFIG_RCU_STRICT_GRACE_PERIOD=y". The current changes cause this scenario to process more than double the number of RCU grace periods per unit time, so who knows?
I am not reporting this one yet because it is of course quite possible that it is due to my changes being messed up.
Thanx, Paul
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[ 99.769536] ------------[ cut here ]------------ [ 99.770052] WARNING: CPU: 5 PID: 36 at kernel/irq_work.c:95 irq_work_queue_on+0x73/0x90 [ 99.770905] Modules linked in: [ 99.771239] CPU: 5 PID: 36 Comm: migration/5 Not tainted 5.8.0-rc3+ #2153 [ 99.771964] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1ubuntu1 04/01/2014 [ 99.772898] RIP: 0010:irq_work_queue_on+0x73/0x90 [ 99.773405] Code: 89 ee 89 df e8 fe 4a fa ff bf 01 00 00 00 e8 24 32 f3 ff 65 8b 05 6d 48 6b 59 85 c0 ba 01 00 00 00 75 b6 e8 16 f9 e9 ff eb af <0f> 0b eb 9d 48 89 ef e8 b1 fe ff ff eb d1 0f 0b eb c3 90 66 2e 0f [ 99.775384] RSP: 0018:ffffae4200197d58 EFLAGS: 00010002 [ 99.775944] RAX: 0000000000000005 RBX: 0000000000000005 RCX: 0000000000000001 [ 99.776708] RDX: 0000000000000000 RSI: 0000000000000005 RDI: ffff96fe9f36a270 [ 99.777466] RBP: ffff96fe9f36a270 R08: 00000019b750dcec R09: 0000000000000000 [ 99.778224] R10: 0000000000000008 R11: ffffffffa7c550a8 R12: 0000000000000046 [ 99.778981] R13: 0000000000000000 R14: 0000000000000000 R15: ffff96fe9ee51580 [ 99.779740] FS: 0000000000000000(0000) GS:ffff96fe9f340000(0000) knlGS:0000000000000000 [ 99.780694] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 99.781318] CR2: 00000000ffffffff CR3: 000000000fe0a000 CR4: 00000000000006e0 [ 99.782076] Call Trace: [ 99.782356] __rcu_read_unlock+0x118/0x140 [ 99.782802] sched_cpu_dying+0x157/0x230 [ 99.783228] ? sched_cpu_starting+0x30/0x30 [ 99.783678] cpuhp_invoke_callback+0x81/0x610 [ 99.784159] ? cpu_disable_common+0x28f/0x2b0 [ 99.784630] take_cpu_down+0x62/0xa0 [ 99.785018] multi_cpu_stop+0x5f/0xe0 [ 99.785417] ? stop_machine_yield+0x10/0x10 [ 99.785869] cpu_stopper_thread+0x80/0x100 [ 99.786310] ? sort_range+0x20/0x20 [ 99.786689] smpboot_thread_fn+0x199/0x230 [ 99.787131] kthread+0x139/0x160 [ 99.787484] ? kthread_park+0x80/0x80 [ 99.787881] ret_from_fork+0x22/0x30 [ 99.788278] ---[ end trace 0b90671b542e1746 ]---
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