Messages in this thread | | | Date | Mon, 28 Oct 2019 18:30:32 +0100 | From | Christian Brauner <> | Subject | Re: [PATCH cgroup/for-5.5] cgroup: remove cgroup_enable_task_cg_lists() optimization |
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On Mon, Oct 28, 2019 at 05:48:52PM +0100, Oleg Nesterov wrote: > On 10/25, Christian Brauner wrote: > > > > On Fri, Oct 25, 2019 at 05:52:25PM +0200, Oleg Nesterov wrote: > > > On 10/25, Christian Brauner wrote: > > > > > > > > On Fri, Oct 25, 2019 at 04:13:25PM +0200, Oleg Nesterov wrote: > > > > > Almost every usage of task->flags (load or sore) can be reported as "data race". > > > > > > > > > > Say, you do > > > > > > > > > > if (task->flags & PF_KTHREAD) > > > > > > > > > > while this task does > > > > > > > > > > current->flags |= PF_FREEZER_SKIP; > > > > > schedule(). > > > > > > > > > > this is data race. > > > > > > > > Right, but I thought we agreed on WONTFIX in those scenarios? > > > > The alternative is to READ_ONCE()/WRITE_ONCE() all of these. > > > > > > Well, in my opinion this is WONTFIX, but I won't argue if someone > > > adds _ONCE to all of these. Same for task->state, exit_state, and > > > more. > > > > Well, I honestly think that state and exit_state would make sense. > > Heh. Again, I am not arguing, but... > > OK, lets suppose we blindly add READ_ONCE() to every access of > task->state/exit_state. > > Yes, this won't hurt and possibly can fix some bugs we are not aware of.
I wasn't planning or working on adding *_ONCE everywhere. ;) I just think it makes sense as a preemptive strike since they are shared (though mostly protected by locks anyway).
> > However, > > > There already were issues that got fixed for example in 3245d6acab98 > > ("exit: fix race between wait_consider_task() and wait_task_zombie()") > > The change above can't fix the problem like this.
No argument about the code we discussed right here, for sure!
> > It is not that this code lacked READ_ONCE(). I am sure me and others > understood that this code can read ->exit_state more than once, just > nobody noticed that in this case this is really wrong. > > IOW, if we simply change the code before 3245d6acab98 to use READ_ONCE() > the code will be equally wrong, and > > > and as far as I understand this would also help kcsan to better detect > > races. > > this change will simply hide the problem from kcsan.
I can't speak to that since the claim that read_once() helps them even if it's not really doing anything. But maybe I misunderstood the k{c,t}san manpage.
Christian
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