The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: b55945c500c5723992504aa03b362fab416863a6
Gitweb: https://git.kernel.org/tip/b55945c500c5723992504aa03b362fab416863a6
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 23 Oct 2024 11:36:41 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 23 Oct 2024 20:52:26 +02:00
sched: Fix pick_next_task_fair() vs try_to_wake_up() race
Syzkaller robot reported KCSAN tripping over the
ASSERT_EXCLUSIVE_WRITER(p->on_rq) in __block_task().
The report noted that both pick_next_task_fair() and try_to_wake_up()
were concurrently trying to write to the same p->on_rq, violating the
assertion -- even though both paths hold rq->__lock.
The logical consequence is that both code paths end up holding a
different rq->__lock. And looking through ttwu(), this is possible
when the __block_task() 'p->on_rq = 0' store is visible to the ttwu()
'p->on_rq' load, which then assumes the task is not queued and
continues to migrate it.
Rearrange things such that __block_task() releases @p with the store
and no code thereafter will use @p again.
Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Reported-by: syzbot+0ec1e96c2cdf5c0e512a@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Marco Elver <elver@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20241023093641.GE16066@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
kernel/sched/fair.c | 21 ++++++++++++++-------
kernel/sched/sched.h | 34 ++++++++++++++++++++++++++++++++--
2 files changed, 46 insertions(+), 9 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c157d48..8796146 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5625,8 +5625,9 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
struct sched_entity *se = pick_eevdf(cfs_rq);
if (se->sched_delayed) {
dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
- SCHED_WARN_ON(se->sched_delayed);
- SCHED_WARN_ON(se->on_rq);
+ /*
+ * Must not reference @se again, see __block_task().
+ */
return NULL;
}
return se;
@@ -7176,7 +7177,11 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
/* Fix-up what dequeue_task_fair() skipped */
hrtick_update(rq);
- /* Fix-up what block_task() skipped. */
+ /*
+ * Fix-up what block_task() skipped.
+ *
+ * Must be last, @p might not be valid after this.
+ */
__block_task(rq, p);
}
@@ -7193,12 +7198,14 @@ static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
util_est_dequeue(&rq->cfs, p);
- if (dequeue_entities(rq, &p->se, flags) < 0) {
- util_est_update(&rq->cfs, p, DEQUEUE_SLEEP);
+ util_est_update(&rq->cfs, p, flags & DEQUEUE_SLEEP);
+ if (dequeue_entities(rq, &p->se, flags) < 0)
return false;
- }
- util_est_update(&rq->cfs, p, flags & DEQUEUE_SLEEP);
+ /*
+ * Must not reference @p after dequeue_entities(DEQUEUE_DELAYED).
+ */
+
hrtick_update(rq);
return true;
}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 081519f..9f9d1cc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2769,8 +2769,6 @@ static inline void sub_nr_running(struct rq *rq, unsigned count)
static inline void __block_task(struct rq *rq, struct task_struct *p)
{
- WRITE_ONCE(p->on_rq, 0);
- ASSERT_EXCLUSIVE_WRITER(p->on_rq);
if (p->sched_contributes_to_load)
rq->nr_uninterruptible++;
@@ -2778,6 +2776,38 @@ static inline void __block_task(struct rq *rq, struct task_struct *p)
atomic_inc(&rq->nr_iowait);
delayacct_blkio_start();
}
+
+ ASSERT_EXCLUSIVE_WRITER(p->on_rq);
+
+ /*
+ * The moment this write goes through, ttwu() can swoop in and migrate
+ * this task, rendering our rq->__lock ineffective.
+ *
+ * __schedule() try_to_wake_up()
+ * LOCK rq->__lock LOCK p->pi_lock
+ * pick_next_task()
+ * pick_next_task_fair()
+ * pick_next_entity()
+ * dequeue_entities()
+ * __block_task()
+ * RELEASE p->on_rq = 0 if (p->on_rq && ...)
+ * break;
+ *
+ * ACQUIRE (after ctrl-dep)
+ *
+ * cpu = select_task_rq();
+ * set_task_cpu(p, cpu);
+ * ttwu_queue()
+ * ttwu_do_activate()
+ * LOCK rq->__lock
+ * activate_task()
+ * STORE p->on_rq = 1
+ * UNLOCK rq->__lock
+ *
+ * Callers must ensure to not reference @p after this -- we no longer
+ * own it.
+ */
+ smp_store_release(&p->on_rq, 0);
}
extern void activate_task(struct rq *rq, struct task_struct *p, int flags);
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