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    SubjectRe: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way
    On 05/11, 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().

    Yes sure. Quoting myself,
    > true->false means daemonize() or do_exit(), seems harmless.
    > false->true means exec from kernel space. That is why FREEZER_KERNEL_THREADS
    > in fact means all tasks, not only kernel threads.

    > 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.

    This is another story, I can't comment because I am not educated enough.

    However, in my opininon THAT PATCH has nothing to do with this problem.
    It just improves the code that we already have.

    > > 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.

    Note the "in that particular case".

    > See? The locking was pointless. Exactly because you release the lock
    > before the user can actually do anything about the return value!

    Yes. See the "Quoting myself" above.

    > Anyway, I think the whole freezer thing is broken. There's no reason to
    > freeze kernel threads.

    It is not perfect. Rafael tries to improve it.

    Do we need freezer? Should we freeze kernel threads? I can't judge. I tried
    to read a long thread about suspend, and failed to understand it.

    I personally think we can simplify things if CPU-hotplug use freezer, at least.


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