Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Wed, Feb 12, 2020 at 12:41 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >> >> On Wed, Feb 12, 2020 at 08:38:33PM +0000, Al Viro wrote: >> > >> > Wait, I thought the whole point of that had been to allow multiple >> > procfs instances for the same userns? Confused... >> >> s/userns/pidns/, sorry > > Right, but we still hold the ref to it here... > > [ Looks more ] > > Oooh. No we don't. Exactly because we don't hold the lock, only the > rcu lifetime, the ref can go away from under us. I see what your > concern is. > > Ouch, this is more painful than I expected - the code flow looked so > simple. I really wanted to avoid a new lock during process shutdown, > because that has always been somewhat painful. The good news is proc_flush_task isn't exactly called from process exit. proc_flush_task is called during zombie clean up. AKA release_task. So proc_flush_task isn't called with any locks held, and it is called in a context where it can sleep. Further after proc_flush_task does it's thing the code goes and does "write_lock_irq(&task_list_lock);" So the code is definitely serialized to one processor already. What would be downside of having a mutex for a list of proc superblocks? A mutex that is taken for both reading and writing the list. Eric
