• Rafael J. Wysocki's avatar
    Freezer: avoid freezing kernel threads prematurely · 0c1eecfb
    Rafael J. Wysocki authored
    Kernel threads should not have TIF_FREEZE set when user space processes are
    being frozen, since otherwise some of them might be frozen prematurely.
    To prevent this from happening we can (1) make exit_mm() unset TIF_FREEZE
    unconditionally just after clearing tsk->mm and (2) make try_to_freeze_tasks()
    check if p->mm is different from zero and PF_BORROWED_MM is unset in p->flags
    when user space processes are to be frozen.
    
    Namely, when user space processes are being frozen, we only should set
    TIF_FREEZE for tasks that have p->mm different from NULL and don't have
    PF_BORROWED_MM set in p->flags.  For this reason task_lock() must be used to
    prevent try_to_freeze_tasks() from racing with use_mm()/unuse_mm(), in which
    p->mm and p->flags.PF_BORROWED_MM are changed under task_lock(p).  Also, we
    need to prevent the following scenario from happening:
    
    * daemonize() is called by a task spawned from a user space code path
    * freezer checks if the task has p->mm set and the result is positive
    * task enters exit_mm() and clears its TIF_FREEZE
    * freezer sets TIF_FREEZE for the task
    * task calls try_to_freeze() and goes to the refrigerator, which is wrong at
      that point
    
    This requires us to acquire task_lock(p) before p->flags.PF_BORROWED_MM and
    p->mm are examined and release it after TIF_FREEZE is set for p (or it turns
    out that TIF_FREEZE should not be set).
    Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
    Cc: Gautham R Shenoy <ego@in.ibm.com>
    Cc: Pavel Machek <pavel@ucw.cz>
    Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
    Cc: Oleg Nesterov <oleg@tv-sign.ru>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0c1eecfb
exit.c 44.2 KB