Messages in this thread | | | Date | Thu, 11 Nov 2010 13:39:29 +0100 | From | Jens Axboe <> | Subject | Re: INFO: suspicious rcu_dereference_check() usage - kernel/pid.c:419 invoked rcu_dereference_check() without protection! |
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On 2010-11-11 13:30, Oleg Nesterov wrote: > 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().
Sure, not a big problem.
> 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.
Indeed...
> Hmm. And copy_io_context() has no callers ;)
Good find. It was previously used by the AS io scheduler, seems there are no users left anymore. I queued up a patch to kill it.
-- Jens Axboe
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