[RFC PATCH 04/21] timers: Use hlist_unhashed() instead of open-coding in timer_pending()

[Will Deacon]

timer_pending() open-codes a version of hlist_unhashed() to check
whether or not the 'timer' parameter has been queued in the timer wheel.
KCSAN detects this as a racy operation and explodes at us:

  | 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

Replace the explicit 'pprev' pointer comparison in timer_pending() with
a call to hlist_unhashed() and initialise the 'expires' timer field
explicitly in do_init_timer() so that the compiler doesn't emit bogus
'maybe used uninitialised' warnings now that it cannot reason statically
about the result of timer_pending().

Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/timer.h | 5 +++--
 kernel/time/timer.c   | 1 +
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 1e6650ed066d..e9610d2988ba 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -158,13 +158,14 @@ static inline void destroy_timer_on_stack(struct timer_list *timer) { }
  *
  * timer_pending will tell whether a given timer is currently pending,
  * or not. Callers must ensure serialization wrt. other operations done
- * to this timer, eg. interrupt contexts, or other CPUs on SMP.
+ * to this timer, eg. interrupt contexts, or other CPUs on SMP if they
+ * cannot tolerate spurious results.
  *
  * return value: 1 if the timer is pending, 0 if not.
  */
 static inline int timer_pending(const struct timer_list * timer)
 {
-	return timer->entry.pprev != NULL;
+	return !hlist_unhashed(&timer->entry);
 }
 
 extern void add_timer_on(struct timer_list *timer, int cpu);
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4820823515e9..9e1c6fc8433a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -780,6 +780,7 @@ static void do_init_timer(struct timer_list *timer,
 			  const char *name, struct lock_class_key *key)
 {
 	timer->entry.pprev = NULL;
+	timer->expires = 0; /* Avoid bogus 'maybe used uninitialized' warning */
 	timer->function = func;
 	timer->flags = flags | raw_smp_processor_id();
 	lockdep_init_map(&timer->lockdep_map, name, key, 0);
-- 
2.20.1