Messages in this thread | | | Date | Tue, 15 Jul 2014 15:12:40 +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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Ah, I am stupid, please ignore.
Of course a TASK_DEAD task can not schedule, but we can race with RUNNING -> DEAD transition. So we should only do put_task_struct() if "prev" was already TASK_DEAD before we drop the rq locks.
On 07/14, Oleg Nesterov wrote: > > On 07/14, Oleg Nesterov wrote: > > > > Yes, the task itself (or, depending ob pov, scheduler) has a reference. > > copy_process() does > > > > /* > > * One for us, one for whoever does the "release_task()" (usually > > * parent) > > */ > > atomic_set(&tsk->usage, 2); > > > > "us" actually means that put_task_struct(TASK_DEAD). > > Off-topic, but I do not understand the huge comment in finish_task_switch(). > Perhaps this all was true a long ago, but currently "prev could be scheduled > on another cpu" is certainly impossible? > > And if it was possible we have much more problems? In particular, in this case > we still can drop the reference twice? > > I'll try to recheck, but do you see anything wrong in the patch below? > > Oleg. > > --- x/kernel/sched/core.c > +++ x/kernel/sched/core.c > @@ -2197,22 +2197,9 @@ static void finish_task_switch(struct rq > __releases(rq->lock) > { > struct mm_struct *mm = rq->prev_mm; > - long prev_state; > > rq->prev_mm = NULL; > > - /* > - * 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. > - * The test for TASK_DEAD must occur while the runqueue locks are > - * still held, otherwise prev could be scheduled on another cpu, die > - * there before we look at prev->state, and then the reference would > - * be dropped twice. > - * Manfred Spraul <manfred@colorfullife.com> > - */ > - prev_state = prev->state; > vtime_task_switch(prev); > finish_arch_switch(prev); > perf_event_task_sched_in(prev, current); > @@ -2222,7 +2209,7 @@ static void finish_task_switch(struct rq > fire_sched_in_preempt_notifiers(current); > if (mm) > mmdrop(mm); > - if (unlikely(prev_state == TASK_DEAD)) { > + if (unlikely(prev->state == TASK_DEAD)) { > if (prev->sched_class->task_dead) > prev->sched_class->task_dead(prev); >
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