Re: Confused about hlist_unhashed_lockless()

[Eric Dumazet]

On Fri, Jan 31, 2020 at 8:43 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> Hi folks,
>
> I just ran into c54a2744497d ("list: Add hlist_unhashed_lockless()")
> but I'm a bit confused about what it's trying to achieve. It also seems
> to have been merged without any callers (even in -next) -- was that
> intentional?
>
> My main source of confusion is the lack of memory barriers. For example,
> if you look at the following pair of functions:
>
>
> static inline int hlist_unhashed_lockless(const struct hlist_node *h)
> {
>         return !READ_ONCE(h->pprev);
> }
>
> static inline void hlist_add_before(struct hlist_node *n,
>                                     struct hlist_node *next)
> {
>         WRITE_ONCE(n->pprev, next->pprev);
>         WRITE_ONCE(n->next, next);
>         WRITE_ONCE(next->pprev, &n->next);
>         WRITE_ONCE(*(n->pprev), n);
> }
>
>
> Then running these two concurrently on the same node means that
> hlist_unhashed_lockless() doesn't really tell you anything about whether
> or not the node is reachable in the list (i.e. there is another node
> with a next pointer pointing to it). In other words, I think all of
> these outcomes are permitted:
>
>         hlist_unhashed_lockless(n)      n reachable in list
>         0                               0 (No reordering)
>         0                               1 (No reordering)
>         1                               0 (No reordering)
>         1                               1 (Reorder first and last WRITE_ONCEs)
>
> So I must be missing some details about the use-case here. Please could
> you enlighten me? The RCU implementation permits only the first three
> outcomes afaict, why not use that and leave non-RCU hlist as it was?
>

I guess the following has been lost :

Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Thu Nov 7 11:23:14 2019 -0800

    timer: use hlist_unhashed_lockless() in timer_pending()

    timer_pending() is mostly used in lockless contexts.

    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@xxxxxxxxxx>
    Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
    Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 1e6650ed066d5d28251b0bd385fc37ef94c96532..0dc19a8c39c9e49a7cde3d34bfa4be8871cbc1c2
100644
--- a/include/linux/timer.h
+++ b/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);
 }