Matt Helsley a écrit : > On Wed, Aug 11, 2010 at 12:18:44AM +0200, Tomasz Buchert wrote: >> Matt Helsley a écrit : >>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote: >>>> Writing 'FROZEN' to freezer.state file does not >>>> forbid the task to be moved away from its cgroup >>>> (for a very short time). Nevertheless the moved task >>>> can become frozen OUTSIDE its cgroup which puts >>>> discussed task in a permanent 'D' state. >>>> >>>> This patch forbids migration of either FROZEN >>>> or FREEZING tasks. >>>> >>>> This behavior was observed and easily reproduced on >>>> a single core laptop. Program and instructions how >>>> to reproduce the bug can be fetched from: >>>> http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c >>> Thanks for the report and the test code. >>> >>> I'm will try to reproduce this race in the next few hours and analyze >>> it since I'm not sure the patch really fixes the race -- it may only >>> make the race trigger less frequently. >>> >>> At the very least the patch won't break the current code since it's >>> essentially a more-strict version of is_task_frozen_enough() -- it lets >>> fewer tasks attach/detach to/from frozen cgroups. >>> >>> Cheers, >>> -Matt Helsley >> Hi Matt! >> I am a novice if it comes to the kernel and I find the cgroup_freezer >> code especially complicated, so definetely this may be not enough to fix >> that. >> Notice also that if you uncomment the line 55 in my testcase this will also >> trigger the race! This, however, makes sense since process may not be in the >> cgroup anymore >> and consequently won't be thawed. > > OK, I triggered it with that. Interesting. >
Good! >> I think that this patch fixes these problems because it does the flag >> checking in a right order: >> first freezing() is used and then frozen() which assures (see >> frozen_process()) that >> the race will not happen. Right? :) > > I see what you mean. It still seems like it wouldn't actually fix the race -- > just make it > harder to trigger. I think you're saying this is what happens without the > patch: > > Time "bug" goes through these states cgroup code checks for these > states > ----------------------------------------------------------------------------------- > | freezing > | is_frozen? Nope. > | frozen > | is_freezing? Nope. > | <move> > V > My first scenario was a bit different: Time "bug" goes through these states cgroup code checks for these states ----------------------------------------------------------------------------------- | freezing | is_task_frozen_enough? Nope. | <move> | frozen V but the problem is the same. > But, without having carefully investigated the details, this could just as > easily happen > with your patch: > > Time "bug" goes through these states cgroup code checks for these > states > ----------------------------------------------------------------------------------- > | is_freezing? Nope. > | is_frozen? Nope. > | freezing > | <move> > | frozen > V > This can't happen as far as I know because there is cgroup_lock around the code in freezer_write() and freezer_can_attach(). The task can't enter 'freezing' state when can_attach is executed. > or: > > Time "bug" goes through these states cgroup code checks for these > states > ----------------------------------------------------------------------------------- > | is_freezing? Nope. > | is_frozen? Nope. > | freezing > | frozen > | <move> > V > Same thing here. > Time "bug" goes through these states cgroup code checks for these > states > ----------------------------------------------------------------------------------- > | is_freezing? Nope. > | freezing > | is_frozen? Nope. > | <move> > | frozen > V > Again. > or: > > Time "bug" goes through these states cgroup code checks for these > states > ----------------------------------------------------------------------------------- > | is_freezing? Nope. > | freezing > | is_frozen? Nope. > | frozen > | <move> > V > > (even with 1 cpu/core) Well, once more. > > Your patch only improves things in the sense that it works for the first > example. We need to prevent the latter cases as well. > > Cheers, > -Matt What do you think? Tomasz _______________________________________________ Containers mailing list [email protected] https://lists.linux-foundation.org/mailman/listinfo/containers _______________________________________________ Devel mailing list [email protected] https://openvz.org/mailman/listinfo/devel
