Messages in this thread | | | Date | Mon, 20 Oct 2014 22:50:06 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign() |
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On 10/21, Kirill Tkhai wrote: > > I think generic helper is a good idea. The prototype looks OK. > > But I'm a little doubt about retry loop. If this helper is generic and > one day it may move to ./include directory,
Well, if we add a generic helper I think it should be exported even if it has a single caller. But I agree this probably needs a justification too.
> isn't there a probability > people will use it wrong? This loop may bring delays or something bad.
Yes, I thought about livelock too. OK, we can remove it, just s/goto retry/return NULL/. Or, perhaps better, return ERR_PTR(-EAGAIN) so that the caller can know that retry is possible.
I do not really mind, and we can reconsider this later. And I will not argue if you prefer to add the rq->curr specific hack (like your patch does).
> And since we still depends on RCU, I'd suggest to add its lockdep assert.
Agreed.
Let me explain what I personally dislike in v3:
- I think that we do not have enough reasons for SLAB_DESTROY_BY_RCU. This is the serious change.
probe_kernel_read() looks better to me, and hopefully IS_ENABLED(DEBUG_PAGEALLOC) can make it conditional.
- PF_EXITING was fine in task_numa_compare(), but if we move this logic into a helper (even if it is not exported) then I think we need a more specific check. sighand == NULL looks better to me because it clearly connects to release_task() which makes this task_struct "rcu-unsafe".
- Again, perhaps we should start we a simple and stupid fix. We can do get_task_struct() under rq->lock or, if nothing else, just
raw_spin_lock_irq(&rq->lock); cur = rq->curr; if (is_idle_task(cur) || (cur->flags & PF_EXITING)) cur = NULL; raw_spin_unlock_irq(&rq->lock);
Either way, I hope you will send v4 ;)
But probably you should wait for for Peter's opinion first.
Oleg.
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