[PATCH] rcu: fix warning in rcu_seq_end()

[Dmitry Vyukov]

rcu_seq_end() increments seq signifying completion of a grace period,
after that checks that the seq is even and wakes
_synchronize_rcu_expedited(). _synchronize_rcu_expedited() uses
wait_event() to wait for even seq. The problem is that wait_event()
can return as soon as seq becomes even without waiting for the wakeup.
In such case the warning in rcu_seq_end() can falsely fire if the next
expedited grace period starts before the check.

Check that seq has good value before incrementing it.

Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: syzkaller@xxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: josh@xxxxxxxxxxxxxxxx
Cc: jiangshanlai@xxxxxxxxx

---

syzkaller-triggered warning:

WARNING: CPU: 0 PID: 4832 at kernel/rcu/tree.c:3533
rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
CPU: 0 PID: 4832 Comm: kworker/0:3 Not tainted 4.10.0+ #276
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Workqueue: events wait_rcu_exp_gp
Call Trace:
 __dump_stack lib/dump_stack.c:15 [inline]
 dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
 panic+0x1fb/0x412 kernel/panic.c:179
 __warn+0x1c4/0x1e0 kernel/panic.c:540
 warn_slowpath_null+0x2c/0x40 kernel/panic.c:583
 rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
 rcu_exp_gp_seq_end kernel/rcu/tree_exp.h:36 [inline]
 rcu_exp_wait_wake+0x8a9/0x1330 kernel/rcu/tree_exp.h:517
 rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:559 [inline]
 wait_rcu_exp_gp+0x83/0xc0 kernel/rcu/tree_exp.h:570
 process_one_work+0xc06/0x1c20 kernel/workqueue.c:2096
 worker_thread+0x223/0x19c0 kernel/workqueue.c:2230
 kthread+0x326/0x3f0 kernel/kthread.c:227
 ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 50fee7689e71..47339b242423 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3531,8 +3531,8 @@ static void rcu_seq_start(unsigned long *sp)
 static void rcu_seq_end(unsigned long *sp)
 {
 	smp_mb(); /* Ensure update-side operation before counter increment. */
+	WARN_ON_ONCE(!(*sp & 0x1));
 	WRITE_ONCE(*sp, *sp + 1);
-	WARN_ON_ONCE(*sp & 0x1);
 }
 
 /* Take a snapshot of the update side's sequence number. */
-- 
2.12.0.rc1.440.g5b76565f74-goog