From 3f1d2a318171bf61850d4e5a72031271e5aada76 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 12 Feb 2014 10:49:30 +0100 Subject: [PATCH] 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 Cc: Ingo Molnar Cc: Steven Rostedt Reported-by: Kirill Tkhai Reported-by: Dan Carpenter Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/20140212094930.GB3545@laptop.programming.kicks-ass.net Signed-off-by: Thomas Gleixner --- kernel/sched/core.c | 18 +++++++++++++++++- kernel/sched/deadline.c | 3 +-- kernel/sched/fair.c | 5 ++--- kernel/sched/idle_task.c | 3 +-- kernel/sched/rt.c | 3 +-- kernel/sched/sched.h | 5 +++++ kernel/sched/stop_task.c | 3 +-- 7 files changed, 28 insertions(+), 12 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index fb9764f..49db434 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4681,6 +4681,22 @@ static void calc_load_migrate(struct rq *rq) atomic_long_add(delta, &calc_load_tasks); } +static void put_prev_task_fake(struct rq *rq, struct task_struct *prev) +{ +} + +static const struct sched_class fake_sched_class = { + .put_prev_task = put_prev_task_fake, +}; + +static struct task_struct fake_task = { + /* + * Avoid pull_{rt,dl}_task() + */ + .prio = MAX_PRIO + 1, + .sched_class = &fake_sched_class, +}; + /* * Migrate all tasks from the rq, sleeping tasks will be migrated by * try_to_wake_up()->select_task_rq(). @@ -4721,7 +4737,7 @@ static void migrate_tasks(unsigned int dead_cpu) if (rq->nr_running == 1) break; - next = pick_next_task(rq, NULL); + next = pick_next_task(rq, &fake_task); BUG_ON(!next); next->sched_class->put_prev_task(rq, next); diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index ed31ef6..bfeb84e 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1008,8 +1008,7 @@ struct task_struct *pick_next_task_dl(struct rq *rq, struct task_struct *prev) if (unlikely(!dl_rq->dl_nr_running)) return NULL; - if (prev) - prev->sched_class->put_prev_task(rq, prev); + put_prev_task(rq, prev); dl_se = pick_next_dl_entity(rq, dl_rq); BUG_ON(!dl_se); diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 40c758b..e884e45 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4690,7 +4690,7 @@ again: if (!cfs_rq->nr_running) goto idle; - if (!prev || prev->sched_class != &fair_sched_class) + if (prev->sched_class != &fair_sched_class) goto simple; /* @@ -4766,8 +4766,7 @@ simple: if (!cfs_rq->nr_running) goto idle; - if (prev) - prev->sched_class->put_prev_task(rq, prev); + put_prev_task(rq, prev); do { se = pick_next_entity(cfs_rq, NULL); diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c index f7d03af..53ff9e7 100644 --- a/kernel/sched/idle_task.c +++ b/kernel/sched/idle_task.c @@ -26,8 +26,7 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl static struct task_struct * pick_next_task_idle(struct rq *rq, struct task_struct *prev) { - if (prev) - prev->sched_class->put_prev_task(rq, prev); + put_prev_task(rq, prev); schedstat_inc(rq, sched_goidle); #ifdef CONFIG_SMP diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 72f9ec7..65c2d68 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1344,8 +1344,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev) if (rt_rq_throttled(rt_rq)) return NULL; - if (prev) - prev->sched_class->put_prev_task(rq, prev); + put_prev_task(rq, prev); p = _pick_next_task_rt(rq); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 92018f9..d276147 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1147,6 +1147,11 @@ struct sched_class { #endif }; +static inline void put_prev_task(struct rq *rq, struct task_struct *prev) +{ + prev->sched_class->put_prev_task(rq, prev); +} + #define sched_class_highest (&stop_sched_class) #define for_each_class(class) \ for (class = sched_class_highest; class; class = class->next) diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c index a4147c9..d6ce65d 100644 --- a/kernel/sched/stop_task.c +++ b/kernel/sched/stop_task.c @@ -31,8 +31,7 @@ pick_next_task_stop(struct rq *rq, struct task_struct *prev) if (!stop || !stop->on_rq) return NULL; - if (prev) - prev->sched_class->put_prev_task(rq, prev); + put_prev_task(rq, prev); stop->se.exec_start = rq_clock_task(rq); -- 2.7.4