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-rw-r--r--kernel/cpuset.c35
1 files changed, 34 insertions, 1 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index ba42b0a76961..12815d3f1a05 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1977,6 +1977,39 @@ void cpuset_fork(struct task_struct *child)
1977 * We don't need to task_lock() this reference to tsk->cpuset, 1977 * We don't need to task_lock() this reference to tsk->cpuset,
1978 * because tsk is already marked PF_EXITING, so attach_task() won't 1978 * because tsk is already marked PF_EXITING, so attach_task() won't
1979 * mess with it, or task is a failed fork, never visible to attach_task. 1979 * mess with it, or task is a failed fork, never visible to attach_task.
1980 *
1981 * Hack:
1982 *
1983 * Set the exiting tasks cpuset to the root cpuset (top_cpuset).
1984 *
1985 * Don't leave a task unable to allocate memory, as that is an
1986 * accident waiting to happen should someone add a callout in
1987 * do_exit() after the cpuset_exit() call that might allocate.
1988 * If a task tries to allocate memory with an invalid cpuset,
1989 * it will oops in cpuset_update_task_memory_state().
1990 *
1991 * We call cpuset_exit() while the task is still competent to
1992 * handle notify_on_release(), then leave the task attached to
1993 * the root cpuset (top_cpuset) for the remainder of its exit.
1994 *
1995 * To do this properly, we would increment the reference count on
1996 * top_cpuset, and near the very end of the kernel/exit.c do_exit()
1997 * code we would add a second cpuset function call, to drop that
1998 * reference. This would just create an unnecessary hot spot on
1999 * the top_cpuset reference count, to no avail.
2000 *
2001 * Normally, holding a reference to a cpuset without bumping its
2002 * count is unsafe. The cpuset could go away, or someone could
2003 * attach us to a different cpuset, decrementing the count on
2004 * the first cpuset that we never incremented. But in this case,
2005 * top_cpuset isn't going away, and either task has PF_EXITING set,
2006 * which wards off any attach_task() attempts, or task is a failed
2007 * fork, never visible to attach_task.
2008 *
2009 * Another way to do this would be to set the cpuset pointer
2010 * to NULL here, and check in cpuset_update_task_memory_state()
2011 * for a NULL pointer. This hack avoids that NULL check, for no
2012 * cost (other than this way too long comment ;).
1980 **/ 2013 **/
1981 2014
1982void cpuset_exit(struct task_struct *tsk) 2015void cpuset_exit(struct task_struct *tsk)
@@ -1984,7 +2017,7 @@ void cpuset_exit(struct task_struct *tsk)
1984 struct cpuset *cs; 2017 struct cpuset *cs;
1985 2018
1986 cs = tsk->cpuset; 2019 cs = tsk->cpuset;
1987 tsk->cpuset = NULL; 2020 tsk->cpuset = &top_cpuset; /* Hack - see comment above */
1988 2021
1989 if (notify_on_release(cs)) { 2022 if (notify_on_release(cs)) {
1990 char *pathbuf = NULL; 2023 char *pathbuf = NULL;