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

[Greg KH]

On Tue, Mar 24, 2020 at 03:36:26PM +0000, Will Deacon wrote:
> 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>

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>