[PATCH] sched/core: Do not touch SMT siblings that have not joined the core yet

[Seiji Nishikawa]

When a CPU comes online, it is added to cpu_smt_mask() of its siblings
before sched_core_cpu_starting() sets its rq->core to the core leader.
For example, on x86, ap_starting() calls set_cpu_sibling_map() first,
and calls notify_cpu_starting() after that. This order is intended.

Before sched_core_cpu_starting() runs, rq->core of the new CPU still
points to its own rq. But rq->core_enabled of the new CPU is already
set. __sched_core_flip() sets it for all possible CPUs, also for
offline CPUs, and CPU offline does not clear it. So __rq_lockp() of the
new CPU returns its own rq->__lock. It does not return the core wide
lock.

If a sibling CPU runs pick_next_task() for SCHED_CORE at this time, it
checks all CPUs in cpu_smt_mask(). It calls update_rq_clock() and
pick_task() for the rq of the new CPU. But it holds only its own core
lock, not the lock of the new CPU. Then lockdep shows this warning in
update_rq_clock().

  debug_locks && !(lock_is_held(&(__rq_lockp(rq))->dep_map) != 0)
  WARNING: kernel/sched/sched.h:1666 at update_rq_clock+0x40a/0xd20 kernel/sched/core.c:875, CPU#0: syz-executor251/5712
  Call Trace:
   <TASK>
   pick_next_task kernel/sched/core.c:6375 [inline]
   __schedule+0x1c69/0x6920 kernel/sched/core.c:7199
   preempt_schedule_irq+0x4e/0x90 kernel/sched/core.c:7606

The next loops in pick_next_task() also change rq_i->core_pick of this
rq, and can call resched_curr() for it, without its lock.
resched_curr() has the same lockdep_assert_rq_held() check.
__sched_core_account_forceidle() also checks all CPUs in the same mask.

CPU offline does not have this problem. remove_siblinginfo() and
sched_core_cpu_dying() both run in take_cpu_down() under stop_machine.
So no sibling CPU can be in pick_next_task() at that time.

Fix this. In these loops, skip a sibling CPU if its rq->core is
different from our rq->core. At run time, rq->core of a CPU is changed
by sched_core_cpu_starting(), sched_core_cpu_deactivate() and
sched_core_cpu_dying(). The first two hold sched_core_lock(), and
sched_core_lock() takes the rq locks of all CPUs in the SMT mask. The
last one runs under stop_machine, as written above. So while we hold
our core lock, the result of this check does not change. A CPU that is
not in the core yet will pick its own tasks. The comment in the
reschedule loop of pick_next_task() already expects this case.

I tested this with the syzbot C reproducer in QEMU. The two vCPUs were
SMT siblings (-smp 2,sockets=1,cores=1,threads=2). Without this patch,
the warning came after about 100 seconds. Just before the warning, a
debug print showed that the loop on CPU 0 saw CPU 1 with rq->core
pointing to CPU 1 itself, and cpu_online() was false for CPU 1. With
this patch, the same state still happened, but there was no warning
in more than 2 hours.

Fixes: 3c474b3239f1 ("sched: Fix Core-wide rq->lock for uninitialized CPUs")
Reported-by: syzbot+681e729b2cf0d4860042@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=681e729b2cf0d4860042
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Seiji Nishikawa <snishika@xxxxxxxxxx>
---
 kernel/sched/core.c       |  7 +++++++
 kernel/sched/core_sched.c |  3 +++
 kernel/sched/sched.h      | 11 +++++++++++
 3 files changed, 21 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1fe40de6ebe3..7f9330e2ef8e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6365,6 +6365,8 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 	max = NULL;
 	for_each_cpu_wrap(i, smt_mask, cpu) {
 		rq_i = cpu_rq(i);
+		if (!sched_core_sibling(rq, rq_i))
+			continue;
 
 		/*
 		 * Current cpu always has its clock updated on entrance to
@@ -6398,6 +6400,9 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 	 */
 	for_each_cpu(i, smt_mask) {
 		rq_i = cpu_rq(i);
+		if (!sched_core_sibling(rq, rq_i))
+			continue;
+
 		p = rq_i->core_pick;
 
 		if (!cookie_equals(p, cookie)) {
@@ -6444,6 +6449,8 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 	 */
 	for_each_cpu(i, smt_mask) {
 		rq_i = cpu_rq(i);
+		if (!sched_core_sibling(rq, rq_i))
+			continue;
 
 		/*
 		 * An online sibling might have gone offline before a task
diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
index 43e0bde3038e..b21eade02c23 100644
--- a/kernel/sched/core_sched.c
+++ b/kernel/sched/core_sched.c
@@ -275,6 +275,9 @@ void __sched_core_account_forceidle(struct rq *rq)
 
 	for_each_cpu(i, smt_mask) {
 		rq_i = cpu_rq(i);
+		if (!sched_core_sibling(rq, rq_i))
+			continue;
+
 		p = rq_i->core_pick ?: rq_i->curr;
 
 		if (p == rq_i->idle)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e656c7059bf8..c65d9cb8ad2e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1513,6 +1513,17 @@ static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
 	return &rq->__lock;
 }
 
+/*
+ * A CPU coming online is set in cpu_smt_mask() before
+ * sched_core_cpu_starting() links its rq->core to the core leader.
+ * Until then it does not share the core-wide lock, so it must not be
+ * touched as part of the core.
+ */
+static inline bool sched_core_sibling(struct rq *rq, struct rq *rq_i)
+{
+	return rq_i->core == rq->core;
+}
+
 extern bool
 cfs_prio_less(const struct task_struct *a, const struct task_struct *b, bool fi);
 
-- 
2.55.0