Messages in this thread | | | Date | Thu, 11 Nov 2010 13:30:15 +0100 | From | Oleg Nesterov <> | Subject | Re: INFO: suspicious rcu_dereference_check() usage - kernel/pid.c:419 invoked rcu_dereference_check() without protection! |
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On 11/11, Jens Axboe wrote: > > On 2010-11-10 17:02, Oleg Nesterov wrote: > > > > But wait. Whatever we do, isn't this code racy? I do not see why, say, > > sys_ioprio_set(IOPRIO_WHO_PROCESS) can't install ->io_context after > > this task has already passed exit_io_context(). > > > > Jens, am I missed something? > > Not sure, I think the original intent was for the tasklist_lock to > protect from a concurrent exit, but that looks like nonsense and it was > just there to protect the task lookup.
Probably. After that (perhaps) there was another reason, see
5b160f5e "copy_process: cosmetic ->ioprio tweak" cf342e52 "Don't need to disable interrupts for tasklist_lock"
But this was dismissed by
fd0928df "ioprio: move io priority from task_struct to io_context"
> How about moving the ->io_context check and exit_io_context() in > do_exit() under the task lock? Coupled with a check for PF_EXITING in > set_task_ioprio().
Yes, I thought about this too. The only drawback is that we should take task_lock() unconditionally in exit_io_context().
Btw, in theory get_task_ioprio() is racy too. "ret = p->io_context->ioprio" can lead to use-after-free. Probably needs task_lock() as well.
Hmm. And copy_io_context() has no callers ;)
Oleg.
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