timer: Use hlist_unhashed_lockless() in timer_pending()

The timer_pending() function is mostly used in lockless contexts, so
Without proper annotations, KCSAN might detect a data-race [1].

Using hlist_unhashed_lockless() instead of hand-coding it seems
appropriate (as suggested by Paul E. McKenney).

[1]

BUG: KCSAN: data-race in del_timer / detach_if_pending

write to 0xffff88808697d870 of 8 bytes by task 10 on cpu 0:
 __hlist_del include/linux/list.h:764 [inline]
 detach_timer kernel/time/timer.c:815 [inline]
 detach_if_pending+0xcd/0x2d0 kernel/time/timer.c:832
 try_to_del_timer_sync+0x60/0xb0 kernel/time/timer.c:1226
 del_timer_sync+0x6b/0xa0 kernel/time/timer.c:1365
 schedule_timeout+0x2d2/0x6e0 kernel/time/timer.c:1896
 rcu_gp_fqs_loop+0x37c/0x580 kernel/rcu/tree.c:1639
 rcu_gp_kthread+0x143/0x230 kernel/rcu/tree.c:1799
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88808697d870 of 8 bytes by task 12060 on cpu 1:
 del_timer+0x3b/0xb0 kernel/time/timer.c:1198
 sk_stop_timer+0x25/0x60 net/core/sock.c:2845
 inet_csk_clear_xmit_timers+0x69/0xa0 net/ipv4/inet_connection_sock.c:523
 tcp_clear_xmit_timers include/net/tcp.h:606 [inline]
 tcp_v4_destroy_sock+0xa3/0x3f0 net/ipv4/tcp_ipv4.c:2096
 inet_csk_destroy_sock+0xf4/0x250 net/ipv4/inet_connection_sock.c:836
 tcp_close+0x6f3/0x970 net/ipv4/tcp.c:2497
 inet_release+0x86/0x100 net/ipv4/af_inet.c:427
 __sock_release+0x85/0x160 net/socket.c:590
 sock_close+0x24/0x30 net/socket.c:1268
 __fput+0x1e1/0x520 fs/file_table.c:280
 ____fput+0x1f/0x30 fs/file_table.c:313
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12060 Comm: syz-executor.5 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine,
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
[ paulmck: Pulled in Eric's later amendments. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/timer.h

@@ -164,7 +164,7 @@ static inline void destroy_timer_on_stack(struct timer_list *timer) { }
  */
 static inline int timer_pending(const struct timer_list * timer)
 {
-	return timer->entry.pprev != NULL;
+	return !hlist_unhashed_lockless(&timer->entry);
 }
 
 extern void add_timer_on(struct timer_list *timer, int cpu);

kernel/time/timer.c

@@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct timer_list *timer,
 
 #define MOD_TIMER_PENDING_ONLY		0x01
 #define MOD_TIMER_REDUCE		0x02
+#define MOD_TIMER_NOTPENDING		0x04
 
 static inline int
 __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int options)
@@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 	 * the timer is re-modified to have the same timeout or ends up in the
 	 * same array bucket then just return:
 	 */
-	if (timer_pending(timer)) {
+	if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) {
 		/*
 		 * The downside of this optimization is that it can result in
 		 * larger granularity than you would get from adding a new
@@ -1133,7 +1134,7 @@ EXPORT_SYMBOL(timer_reduce);
 void add_timer(struct timer_list *timer)
 {
 	BUG_ON(timer_pending(timer));
-	mod_timer(timer, timer->expires);
+	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
 }
 EXPORT_SYMBOL(add_timer);
 
@@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout)
 
 	timer.task = current;
 	timer_setup_on_stack(&timer.timer, process_timeout, 0);
-	__mod_timer(&timer.timer, expire, 0);
+	__mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING);
 	schedule();
 	del_singleshot_timer_sync(&timer.timer);