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    SubjectRe: [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c
    Anton Vorontsov <> writes:

    > On Mon, Jan 30, 2012 at 05:51:20PM -0800, Eric W. Biederman wrote:
    > [...]
    >> > Yes, in LMK driver we don't need to be accurate. I probably could use
    >> > rcu_read_lock, but the plan was in not holding any global locks (in
    >> > this case the rcu) at all, instead I'd like to hold just a reference
    >> > of the task, which the driver is analyzing at this time. Once we decide
    >> > (to kill or not to kill the task), we either send a signal (and drop
    >> > the reference) or just drop the reference.
    >> rcu_read_lock unless it is implemented wrong is free from a lock
    >> perspective. rcu_read_lock only touches local state.
    >> >From the look of your loop it already does a walk through the entire
    >> process list so it looks to me like playing games with get_task_struct
    >> and put_task_struct are going to be much more expensive.
    >> proc grabs task references because we can't hold the rcu_read_lock
    >> over a copy_to_user because that is a sleeping function.
    >> You don't call anything that sleeps so rcu_read_lock should be
    >> sufficient.
    > I'll just repeat what I told to Paul E. McKenney:
    > [...] the locking part wasn't my concern at all. As I said before,
    > LMK (low memory killer) itself is not important, and we don't care
    > about its overhead, unless it blocks another kernel activity --
    > which is my main concern.
    > So, reader part is not interesting in sense of overhead or
    > efficiency.
    > The interesting questions are:
    > 1. Can the kernel create processes while LMK traverses the list?
    > 2. Can the kernel free processes while LMK traverses the list?
    > Looking into kernel/fork.c:copy_process(), it does this:
    > - Takes a write lock on tasklist_lock;
    > - Uses list_add_tail_rcu() to add a task.
    > So, with current LMK driver (it grabs the tasklist_lock), it seems
    > that the kernel won't able to create processes while LMK traverse the
    > tasks.
    > Looking into kernel/exit.c:release_task(), it does this:
    > - Takes a write lock on tasklist_lock;
    > - Deletes the task from the list using list_del_rcu()
    > - Releases tasklist_lock;
    > - Issues call_rcu(&p->rcu, delayed_put_task_struct), which
    > then actually completely frees the task;
    > So, with the current LMK driver, kernel won't able to release
    > processes while LMK traverse the processes, because LMK takes
    > the tasklist_lock.
    > By using rcu_read_lock() we would solve "1.", i.e. kernel will able
    > to create processes, but we still won't able to free processes (well,
    > for the most part we will, except that we'll only free memory after
    > LMK finishes its traverse).

    Correct. We will only free the task_struct after you release the
    rcu_read_lock. Many of the other resources are freed before the
    task_struct. So most of the memory of a process should be freeable
    even with the rcu_read_lock held.


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