summaryrefslogtreecommitdiffstats
path: root/kernel/sched/sched.h
diff options
context:
space:
mode:
authorKirill Tkhai <ktkhai@parallels.com>2014-10-27 15:40:52 +0100
committerIngo Molnar <mingo@kernel.org>2014-11-04 07:17:50 +0100
commit67dfa1b756f250972bde31d65e3f8fde6aeddc5b (patch)
tree0c7478994f086854b764fdc7474416facb660912 /kernel/sched/sched.h
parentsched: Use WARN_ONCE for the might_sleep() TASK_RUNNING test (diff)
downloadlinux-67dfa1b756f250972bde31d65e3f8fde6aeddc5b.tar.xz
linux-67dfa1b756f250972bde31d65e3f8fde6aeddc5b.zip
sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl()
Currently used hrtimer_try_to_cancel() is racy: raw_spin_lock(&rq->lock) ... dl_task_timer raw_spin_lock(&rq->lock) ... raw_spin_lock(&rq->lock) ... switched_from_dl() ... ... hrtimer_try_to_cancel() ... ... switched_to_fair() ... ... ... ... ... ... ... ... raw_spin_unlock(&rq->lock) ... (asquired) ... ... ... ... ... ... do_exit() ... ... schedule() ... ... raw_spin_lock(&rq->lock) ... raw_spin_unlock(&rq->lock) ... ... ... raw_spin_unlock(&rq->lock) ... raw_spin_lock(&rq->lock) ... ... (asquired) put_task_struct() ... ... free_task_struct() ... ... ... ... raw_spin_unlock(&rq->lock) ... (asquired) ... ... ... ... ... (use after free) ... So, let's implement 100% guaranteed way to cancel the timer and let's be sure we are safe even in very unlikely situations. rq unlocking does not limit the area of switched_from_dl() use, because this has already been possible in pull_dl_task() below. Let's consider the safety of of this unlocking. New code in the patch is working when hrtimer_try_to_cancel() fails. This means the callback is running. In this case hrtimer_cancel() is just waiting till the callback is finished. Two 1) Since we are in switched_from_dl(), new class is not dl_sched_class and new prio is not less MAX_DL_PRIO. So, the callback returns early; it's right after !dl_task() check. After that hrtimer_cancel() returns back too. The above is: raw_spin_lock(rq->lock); ... ... dl_task_timer() ... raw_spin_lock(rq->lock); switched_from_dl() ... hrtimer_try_to_cancel() ... raw_spin_unlock(rq->lock); ... hrtimer_cancel() ... ... raw_spin_unlock(rq->lock); ... return HRTIMER_NORESTART; ... ... raw_spin_lock(rq->lock); ... 2) But the below is also possible: dl_task_timer() raw_spin_lock(rq->lock); ... raw_spin_unlock(rq->lock); raw_spin_lock(rq->lock); ... switched_from_dl() ... hrtimer_try_to_cancel() ... ... return HRTIMER_NORESTART; raw_spin_unlock(rq->lock); ... hrtimer_cancel(); ... raw_spin_lock(rq->lock); ... In this case hrtimer_cancel() returns immediately. Very unlikely case, just to mention. Nobody can manipulate the task, because check_class_changed() is always called with pi_lock locked. Nobody can force the task to participate in (concurrent) priority inheritance schemes (the same reason). All concurrent task operations require pi_lock, which is held by us. No deadlocks with dl_task_timer() are possible, because it returns right after !dl_task() check (it does nothing). If we receive a new dl_task during the time of unlocked rq, we just don't have to do pull_dl_task() in switched_from_dl() further. Signed-off-by: Kirill Tkhai <ktkhai@parallels.com> [ Added comments] Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Juri Lelli <juri.lelli@arm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1414420852.19914.186.camel@tkhai Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/sched/sched.h')
-rw-r--r--kernel/sched/sched.h5
1 files changed, 5 insertions, 0 deletions
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ec3917c5f898..49b941fe2cc2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1157,6 +1157,11 @@ struct sched_class {
void (*task_fork) (struct task_struct *p);
void (*task_dead) (struct task_struct *p);
+ /*
+ * The switched_from() call is allowed to drop rq->lock, therefore we
+ * cannot assume the switched_from/switched_to pair is serliazed by
+ * rq->lock. They are however serialized by p->pi_lock.
+ */
void (*switched_from) (struct rq *this_rq, struct task_struct *task);
void (*switched_to) (struct rq *this_rq, struct task_struct *task);
void (*prio_changed) (struct rq *this_rq, struct task_struct *task,