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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>