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    SubjectRe: [PATCH 03/10] threadgroup: extend threadgroup_lock() to cover exit and exec
    On Sun, Nov 13, 2011 at 05:44:32PM +0100, Frederic Weisbecker wrote:
    > On Tue, Nov 01, 2011 at 04:46:26PM -0700, Tejun Heo wrote:
    > > threadgroup_lock() protected only protected against new addition to
    > > the threadgroup, which was inherently somewhat incomplete and
    > > problematic for its only user cgroup. On-going migration could race
    > > against exec and exit leading to interesting problems - the symmetry
    > > between various attach methods, task exiting during method execution,
    > > ->exit() racing against attach methods, migrating task switching basic
    > > properties during exec and so on.
    > >
    > > This patch extends threadgroup_lock() such that it protects against
    > > all three threadgroup altering operations - fork, exit and exec. For
    > > exit, threadgroup_change_begin/end() calls are added to exit path.
    > > For exec, threadgroup_[un]lock() are updated to also grab and release
    > > cred_guard_mutex.
    > >
    > > With this change, threadgroup_lock() guarantees that the target
    > > threadgroup will remain stable - no new task will be added, no new
    > > PF_EXITING will be set and exec won't happen.
    > >
    > > The next patch will update cgroup so that it can take full advantage
    > > of this change.
    > I don't want to nitpick really, but IMHO the races involved in exit and exec
    > are too different, specific and complicated on their own to be solved in a
    > single one patch. This should be split in two things.
    > The specific problems and their fix need to be described more in detail
    > in the changelog because the issues are very tricky.
    > The exec case:
    > IIUC, the race in exec is about the group leader that can be changed
    > to become the exec'ing thread, making while_each_thread() unsafe.
    > We also have other things happening there like all the other threads
    > in the group that get killed, but that should be handled by the threadgroup_change_begin()
    > you put in the exit path.
    > The old leader is also killed but release_task() -> __unhash_process() is called
    > for it manually from the exec path. However this thread too should be covered by your
    > synchronisation in exit().
    > So after your protection in the exit path, the only thing to protect against in exec
    > is that group_leader that can change concurrently. But I may be missing something in the picture.
    > Also note this is currently protected by the tasklist readlock. Cred guard mutex is
    > certainly better, I just don't remember if you remove the tasklist lock in a
    > further patch.

    Ah recalling what Ben Blum said, we also need the leader to stay stable because it
    is excpected to be passed in ->can_attach(), ->attach(), ->cancel_attach(), ...
    Although that's going to change after your patches that pass a flex array.

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