Messages in this thread | | | Date | Wed, 18 Jan 2012 10:42:19 +0100 | From | Ingo Molnar <> | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition |
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* Oleg Nesterov <oleg@redhat.com> wrote:
> On 01/17, Ingo Molnar wrote: > > > > * Yasunori Goto <y-goto@jp.fujitsu.com> wrote: > > > > > --- linux-3.2.orig/kernel/exit.c > > > +++ linux-3.2/kernel/exit.c > > > @@ -1038,6 +1038,22 @@ NORET_TYPE void do_exit(long code) > > > > > > preempt_disable(); > > > exit_rcu(); > > > + > > > + /* > > > + * The setting of TASK_RUNNING by try_to_wake_up() may be delayed > > > + * when the following two conditions become true. > > > + * - There is race condition of mmap_sem (It is acquired by > > > + * exit_mm()), and > > > + * - SMI occurs before setting TASK_RUNINNG. > > > + * (or hypervisor of virtual machine switches to other guest) > > > + * As a result, we may become TASK_RUNNING after becoming TASK_DEAD > > > + * > > > + * To avoid it, we have to wait for releasing tsk->pi_lock which > > > + * is held by try_to_wake_up() > > > + */ > > > + smp_mb(); > > > + raw_spin_unlock_wait(&tsk->pi_lock); > > > > Hm, unlock_wait() is really nasty. Wouldnt the adoption of > > the -rt kernel's delayed task put logic solve most of these > > races? > > How? The problem is that the exiting task can do the last > schedule() in TASK_RUNNING state, this breaks the TASK_DEAD > logic in finish_task_switch().
Well, but does the -rt kernel suffer from the same race? It can generate delays at the exact same place, and can generate much longer delays than an SMI, if a high-prio RT task comes along.
So if there's something in the -rt kernel that fixes this race we'd like to have that. If the bug is present in the -rt kernel then why didn't it ever get triggered? We caught much more narrow races in -rt, and very early on in the project.
Thanks,
Ingo
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