On Saturday, 12 May 2007 02:08, Linus Torvalds wrote: > > On Sat, 12 May 2007, Oleg Nesterov wrote: > > > > things change, ->mm is not stable if the kernel thread does use_mm/unuse_mm. > > ->mm is not stable *regardless*! > > Trivial examples: > - kernel thread does execve() > - user thread does exit(). > > The use "use_mm()" and "unuse_mm()" things are total red herrings. > > If the freezer depends on the difference between user and kernel threads, > then THAT PATCH IS BUGGY. It's that simple. It tests something that simply > isn't stable outside the lock, and then returns that value after having > unlocked it. > > It might as well return a random number. > > > However, the return value == 0 does not change in that particular case, > > exactly because is_user_space() takes task_lock(). > > As does exit_mm() etc. > > That's NOT THE POINT. You cannot use the end result after releasing the > task lock, because the moment you release the task lock, it becomes > totally irrelevant, and may not be true any more. > > Example (a): > - you ask "is_user_space(p)", it returns 1. > - before you actually have time to do anything about it, the task exists, > and (since you don't hold the lock any more) will now have a NULL > tsk->mm again (and would now return 0 if you called it again).
In which case we won't be freezing this task at all. > Example (b): > - you ask "is_user_space(p)" and it returns 0, because it's a kernel > thread > - before you actually do anything about it (but after you released the > task lock), the kernel thread does an "execve(/sbin/hotplug)" and is no > longer a kernel thread. This is a special case that needs special handling. > In both cases will the caller have a return value THAT IS NO LONGER TRUE. > > See? The locking was pointless. Exactly because you release the lock > before the user can actually do anything about the return value! > > The fact that the locking protects against the very specific case of AIO > where the threads _stay_ user tasks and don't really change is pretty much > irrelevant, as far as I can see. Well, I disagree. We need the locking *exactly* to avoid situations in which the threads don't really change, but we might think that they *have changed*. More precisely, it's needed, because without it kernel threads which execute use_mm()/unuse_mm() might be identified as user space processes, and that would be wrong. The other cases are beyond the scope of this patch. Rafael - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
