Commit 06fed338 authored by Paul Jackson's avatar Paul Jackson Committed by Linus Torvalds
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[PATCH] cpuset: oops in exit on null cpuset fix



Fix a latent bug in cpuset_exit() handling.  If a task tried to allocate
memory after calling cpuset_exit(), it oops'd in
cpuset_update_task_memory_state() on a NULL cpuset pointer.

So set the exiting tasks cpuset to the root cpuset instead of to NULL.

A distro kernel hit this with an added kernel package that had just such a
hook (allocating memory) in the exit code path.
Signed-off-by: default avatarPaul Jackson <pj@sgi.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 651c29a1
......@@ -1977,6 +1977,39 @@ void cpuset_fork(struct task_struct *child)
* We don't need to task_lock() this reference to tsk->cpuset,
* because tsk is already marked PF_EXITING, so attach_task() won't
* mess with it, or task is a failed fork, never visible to attach_task.
*
* Hack:
*
* Set the exiting tasks cpuset to the root cpuset (top_cpuset).
*
* Don't leave a task unable to allocate memory, as that is an
* accident waiting to happen should someone add a callout in
* do_exit() after the cpuset_exit() call that might allocate.
* If a task tries to allocate memory with an invalid cpuset,
* it will oops in cpuset_update_task_memory_state().
*
* We call cpuset_exit() while the task is still competent to
* handle notify_on_release(), then leave the task attached to
* the root cpuset (top_cpuset) for the remainder of its exit.
*
* To do this properly, we would increment the reference count on
* top_cpuset, and near the very end of the kernel/exit.c do_exit()
* code we would add a second cpuset function call, to drop that
* reference. This would just create an unnecessary hot spot on
* the top_cpuset reference count, to no avail.
*
* Normally, holding a reference to a cpuset without bumping its
* count is unsafe. The cpuset could go away, or someone could
* attach us to a different cpuset, decrementing the count on
* the first cpuset that we never incremented. But in this case,
* top_cpuset isn't going away, and either task has PF_EXITING set,
* which wards off any attach_task() attempts, or task is a failed
* fork, never visible to attach_task.
*
* Another way to do this would be to set the cpuset pointer
* to NULL here, and check in cpuset_update_task_memory_state()
* for a NULL pointer. This hack avoids that NULL check, for no
* cost (other than this way too long comment ;).
**/
void cpuset_exit(struct task_struct *tsk)
......@@ -1984,7 +2017,7 @@ void cpuset_exit(struct task_struct *tsk)
struct cpuset *cs;
cs = tsk->cpuset;
tsk->cpuset = NULL;
tsk->cpuset = &top_cpuset; /* Hack - see comment above */
if (notify_on_release(cs)) {
char *pathbuf = NULL;
......
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