Messages in this thread | | | Date | Tue, 29 Jun 2010 19:54:22 +0200 | From | Oleg Nesterov <> | Subject | Re: while_each_thread() under rcu_read_lock() is broken? |
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On 06/29, Paul E. McKenney wrote: > > On Tue, Jun 29, 2010 at 03:05:03PM +0200, Oleg Nesterov wrote: > > > > Paul, please let me know if I misunderstood your concerns, or if I missed > > something. > > Thank you very much for laying this out completely! I was having a hard > time believing that it was OK to miss threads in the "ls /proc/2910/task" > case. But of course similar issues can arise when running "ls" on a > directory with lots of files that are coming and going quickly in the > meantime, I guess.
Yes. And again, even if 2910 is not the group leader and it is exiting, "ls /proc/2910/task" will work because proc_task_readdir() akways starts at 2910->group_leader == 2008.
It doesn't work only if proc_task_readdir() can't find its leader, in this particular case this just means 2910 no longer exists, and thus /proc/2910/ is dead even if we can still find this dentry.
> And if proc_task_fill_cache() fails, we can miss > tasks as well, correct?
Well, yes and no.
Sure, if proc_task_fill_cache() fails we didn't reported all threads. But if /bin/ls does readdir() again after that, proc_task_readdir() tries to contunue starting from the last-pid-we-failed-to-report. If there is no task with that pid, we start from the group_leader and skip the number-of-already-reported-threads.
So, we have a lot of issues here, we can miss some thread because "skip the number-of-already-reported-threads" can't be really accurate.
But, to clarify, this has almost nothing to do with the original problem. Afaics, if we change first_tid() to use next_thread_careful() instead of next_thread(), we close the pure-theoretical race with exec but that is all. (and I am still not sure this race does exist, and even if it does we can fix it without next_thread_careful).
> Given all this, I believe that your fix really does work.
Great. I'll send the patch once I inspect zap_threads() and current_is_single_threaded() to figure out which changes they need.
Oleg.
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