Messages in this thread | | | Date | Thu, 24 Jun 2010 20:42:36 -0700 | From | "Paul E. McKenney" <> | Subject | Re: while_each_thread() under rcu_read_lock() is broken? |
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On Thu, Jun 24, 2010 at 05:08:10PM -0700, Eric W. Biederman wrote: > Oleg Nesterov <oleg@redhat.com> writes: > > > On 06/24, Chris Friesen wrote: > >> > >> On 06/24/2010 12:07 PM, Paul E. McKenney wrote: > >> > >> > 3. The thread-group leader might do pthread_exit(), removing itself > >> > from the thread group -- and might do so while the hapless reader > >> > is referencing that thread. > >> > > >> > But isn't this prohibited? Or is it really legal to do a > >> > pthread_create() to create a new thread and then have the > >> > parent thread call pthread_exit()? Not something I would > >> > consider trying in my own code! Well, I might, just to > >> > be perverse, but... ;-) > >> > >> I believe SUS allows the main thread to explicitly call pthread_exit(), > >> leaving the other threads to run. If the main() routine just returns > >> then it implicitly calls exit(). > > > > Correct. > > > > But, to clarify, if the main thread does pthread_exit() (sys_exit, > > actually), it won't be removed from the group. It will be zombie > > until all other threads exit. > > That we don't cleanup that zombie leaders is unfortunate really, it > means we have the entire de_thread special case. But short fixing > libpthread to not make bad assumptions there is little we can do about > it really.
Keeping the zombie leaders does make at least one of the lockless scan cases quite a bit simpler. I think, anyway.
> I'm only half following this conversation. > > If what we are looking for is a stable list_head that won't disappear > on us we should be able to put one in sighand_struct or signal_struct > (I forget which is which at the moment) and have a list_head that > lives for the life of the longest living thread, and that won't get > messed up by things like de_thread, and then next_thread could simply > return NULL when we hit the end of the list.
Oleg did suggest this possibility, but there were complications that I do not claim to fully understand.
Thanx, Paul
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