Messages in this thread | | | Date | Tue, 29 Jul 2014 17:53:09 +0200 | From | Oleg Nesterov <> | Subject | Re: finish_task_switch && prev_state (Was: sched, timers: use after free in __lock_task_sighand when exiting a process) |
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On 07/29, Peter Zijlstra wrote: > > On Tue, Jul 29, 2014 at 11:10:18AM +0200, Peter Zijlstra wrote: > > On Tue, Jul 15, 2014 at 04:25:25PM +0200, Oleg Nesterov wrote: > > > > > And probably I missed something again, but it seems that this logic is broken > > > with __ARCH_WANT_UNLOCKED_CTXSW. > > > > > > Of course, even if I am right this is pure theoretical, but smp_wmb() before > > > "->on_cpu = 0" is not enough and we need a full barrier ? > > > > (long delay there, forgot about this thread, sorry) > > > > Yes, I think I see that.. but now I think the comment is further wrong. > > > > Its not rq->lock that is important, remember, a concurrent wakeup onto > > another CPU does not require our rq->lock at all. > > > > It is the ->on_cpu = 0 store that is important (for both the > > UNLOCKED_CTXSW cases). As soon as that store comes through the task can > > start running on the remote cpu.
Yes, I came to the same conclusion right after I sent that email.
> > Now the below patch 'fixes' this but at the cost of adding a full > > barrier which is somewhat unfortunate to say the least.
And yes, this is obviously the "fix" I had in mind, but:
> > wmb's are free on x86 and generally cheaper than mbs, so it would to > > find another solution to this problem... > > Something like so then?
Hmm, indeed! Unfortunately I didn't find this simple solution. Yes, I think we should check current->state == TASK_DEAD,
> @@ -2304,6 +2293,21 @@ context_switch(struct rq *rq, struct task_struct *prev, > struct task_struct *next) > { > struct mm_struct *mm, *oldmm; > + /* > + * A task struct has one reference for the use as "current". > + * If a task dies, then it sets TASK_DEAD in tsk->state and calls > + * schedule one last time. The schedule call will never return, and > + * the scheduled task must drop that reference. > + * > + * We must observe prev->state before clearing prev->on_cpu (in > + * finish_lock_switch), otherwise a concurrent wakeup can get prev > + * running on another CPU and we could race with its RUNNING -> DEAD > + * transition, and then the reference would be dropped twice. > + * > + * We avoid the race by observing prev->state while it is still > + * current. > + */ > + long prev_state = prev->state;
This doesn't really matter, but probably it would be better to do this right before switch_to(), prev == current until this point.
Oleg.
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