sched: Fix hotplug task migration
authorPeter Zijlstra <peterz@infradead.org>
Wed, 12 Feb 2014 09:49:30 +0000 (10:49 +0100)
committerThomas Gleixner <tglx@linutronix.de>
Fri, 21 Feb 2014 20:43:18 +0000 (21:43 +0100)
commit3f1d2a318171bf61850d4e5a72031271e5aada76
tree58b0fe5519ddeaac88983ff2ba9818f8695db726
parent6e83125c6b151afa139c8852c099d6d92954fe3b
sched: Fix hotplug task migration

Dan Carpenter reported:

> kernel/sched/rt.c:1347 pick_next_task_rt() warn: variable dereferenced before check 'prev' (see line 1338)
> kernel/sched/deadline.c:1011 pick_next_task_dl() warn: variable dereferenced before check 'prev' (see line 1005)

Kirill also spotted that migrate_tasks() will have an instant NULL
deref because pick_next_task() will immediately deref prev.

Instead of fixing all the corner cases because migrate_tasks() can
pass in a NULL prev task in the unlikely case of hot-un-plug, provide
a fake task such that we can remove all the NULL checks from the far
more common paths.

A further problem; not previously spotted; is that because we pushed
pre_schedule() and idle_balance() into pick_next_task() we now need to
avoid those getting called and pulling more tasks on our dying CPU.

We avoid pull_{dl,rt}_task() by setting fake_task.prio to MAX_PRIO+1.
We also note that since we call pick_next_task() exactly the amount of
times we have runnable tasks present, we should never land in
idle_balance().

Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Kirill Tkhai <tkhai@yandex.ru>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140212094930.GB3545@laptop.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
kernel/sched/core.c
kernel/sched/deadline.c
kernel/sched/fair.c
kernel/sched/idle_task.c
kernel/sched/rt.c
kernel/sched/sched.h
kernel/sched/stop_task.c