On 2013/2/5 16:28, Kirill A. Shutemov wrote: > On Tue, Feb 05, 2013 at 11:40:50AM +0800, Li Zefan wrote: >> On 2013/2/4 18:15, Kirill A. Shutemov wrote: >>> On Sat, Feb 02, 2013 at 05:58:58PM +0200, Kirill A. Shutemov wrote: >>>> On Sat, Feb 02, 2013 at 02:50:44PM +0800, Li Zefan wrote: >>>>> When an eventfd is closed, a wakeup with POLLHUP will be issued, >>>>> but cgroup wants to issue wakeup explicitly, so when a cgroup is >>>>> removed userspace can be notified. >>>>> >>>>> Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx> >>> >>> Hm.. Looks like it will break eventfd semantics: >>> >>> 1. One eventfd can be used for deliver more then one notification from >>> one or more cgroups. POLLHUP on removing one of cgroups is not valid. >>> >>> 2. It's valid to have eventfd opened only by one userspace application. We >>> should not close it, just because cgroup is removed. >>> >>> I think problem with multiple threads waiting an event on eventfd should >>> be handled in userspace. >>> >> >> I didn't realize this.. and if a cgroup is removed, the woken thread may not >> be the thread that is waiting on this cgroup. > > Why? The only threads who read() or poll() the eventfd will be wake up, > won't they? Do you have a code sample to demonstrate the issue? > All the threads will be woken up, but one of them will consume the event counter, and then all other threads will keep waiting. >> How crappy.. I don't know how >> userspace is going to deal with all these. >> >> And another bug spotted. We can pass fd of memory.usage_in_bytes of cgroup A >> to cgroup.event_control of cgroup B, and then we won't get memory usage >> notification from A but B! What's worse, if A and B are in different mount >> hierarchy, boom! > > I think we can ignore which cgroup event_control is belong to, and just > use cgroup of cfile as base. It also means you can use one event_control fd > for registering events to different cgroups. It can be handy. > The most reasonal usage is, cgroup.event_control exists in the root cgroup only, and it's used to register events to all cgroups. But I don't think we can change the current interface that each cgroup has a cgroup.event_control, so we'll restrict event registration as my patch does. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
