• Matt Helsley's avatar
    Freezer: Fix buggy resume test for tasks frozen with cgroup freezer · 5a7aadfe
    Matt Helsley authored
    When the cgroup freezer is used to freeze tasks we do not want to thaw
    those tasks during resume. Currently we test the cgroup freezer
    state of the resuming tasks to see if the cgroup is FROZEN.  If so
    then we don't thaw the task. However, the FREEZING state also indicates
    that the task should remain frozen.
    
    This also avoids a problem pointed out by Oren Ladaan: the freezer state
    transition from FREEZING to FROZEN is updated lazily when userspace reads
    or writes the freezer.state file in the cgroup filesystem. This means that
    resume will thaw tasks in cgroups which should be in the FROZEN state if
    there is no read/write of the freezer.state file to trigger this
    transition before suspend.
    
    NOTE: Another "simple" solution would be to always update the cgroup
    freezer state during resume. However it's a bad choice for several reasons:
    Updating the cgroup freezer state is somewhat expensive because it requires
    walking all the tasks in the cgroup and checking if they are each frozen.
    Worse, this could easily make resume run in N^2 time where N is the number
    of tasks in the cgroup. Finally, updating the freezer state from this code
    path requires trickier locking because of the way locks must be ordered.
    
    Instead of updating the freezer state we rely on the fact that lazy
    updates only manage the transition from FREEZING to FROZEN. We know that
    a cgroup with the FREEZING state may actually be FROZEN so test for that
    state too. This makes sense in the resume path even for partially-frozen
    cgroups -- those that really are FREEZING but not FROZEN.
    Reported-by: default avatarOren Ladaan <orenl@cs.columbia.edu>
    Signed-off-by: default avatarMatt Helsley <matthltc@us.ibm.com>
    Cc: stable@kernel.org
    Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
    5a7aadfe
cgroup_freezer.c 9.33 KB