Messages in this thread Patch in this message | | | From | Eric Dumazet <> | Date | Fri, 31 Jan 2020 08:48:05 -0800 | Subject | Re: Confused about hlist_unhashed_lockless() |
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On Fri, Jan 31, 2020 at 8:43 AM Will Deacon <will@kernel.org> 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@google.com> 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@google.com> Cc: "Paul E. McKenney" <paulmck@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de>
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); }
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