• Tejun Heo's avatar
    cgroup: pass around cgroup_subsys_state instead of cgroup in subsystem methods · eb95419b
    Tejun Heo authored
    cgroup is currently in the process of transitioning to using struct
    cgroup_subsys_state * as the primary handle instead of struct cgroup *
    in subsystem implementations for the following reasons.
    
    * With unified hierarchy, subsystems will be dynamically bound and
      unbound from cgroups and thus css's (cgroup_subsys_state) may be
      created and destroyed dynamically over the lifetime of a cgroup,
      which is different from the current state where all css's are
      allocated and destroyed together with the associated cgroup.  This
      in turn means that cgroup_css() should be synchronized and may
      return NULL, making it more cumbersome to use.
    
    * Differing levels of per-subsystem granularity in the unified
      hierarchy means that the task and descendant iterators should behave
      differently depending on the specific subsystem the iteration is
      being performed for.
    
    * In majority of the cases, subsystems only care about its part in the
      cgroup hierarchy - ie. the hierarchy of css's.  Subsystem methods
      often obtain the matching css pointer from the cgroup and don't
      bother with the cgroup pointer itself.  Passing around css fits
      much better.
    
    This patch converts all cgroup_subsys methods to take @css instead of
    @cgroup.  The conversions are mostly straight-forward.  A few
    noteworthy changes are
    
    * ->css_alloc() now takes css of the parent cgroup rather than the
      pointer to the new cgroup as the css for the new cgroup doesn't
      exist yet.  Knowing the parent css is enough for all the existing
      subsystems.
    
    * In kernel/cgroup.c::offline_css(), unnecessary open coded css
      dereference is replaced with local variable access.
    
    This patch shouldn't cause any behavior differences.
    
    v2: Unnecessary explicit cgrp->subsys[] deref in css_online() replaced
        with local variable @css as suggested by Li Zefan.
    
        Rebased on top of new for-3.12 which includes for-3.11-fixes so
        that ->css_free() invocation added by da0a12ca
    
     ("cgroup: fix a
        leak when percpu_ref_init() fails") is converted too.  Suggested
        by Li Zefan.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Acked-by: default avatarLi Zefan <lizefan@huawei.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
    Acked-by: default avatarVivek Goyal <vgoyal@redhat.com>
    Acked-by: default avatarAristeu Rozanski <aris@redhat.com>
    Acked-by: default avatarDaniel Wagner <daniel.wagner@bmw-carit.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Balbir Singh <bsingharora@gmail.com>
    Cc: Matt Helsley <matthltc@us.ibm.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    eb95419b