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Date:   Sat, 9 Nov 2019 10:53:36 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH 1/2] list: add hlist_unhashed_lockless()
Message-ID: <20191109185336.GA19013@paulmck-ThinkPad-P72>
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On Sat, Nov 09, 2019 at 09:54:40AM -0800, Paul E. McKenney wrote:
> On Fri, Nov 08, 2019 at 07:15:16PM -0800, Eric Dumazet wrote:
> > On Fri, Nov 8, 2019 at 3:42 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > >
> > > On Fri, Nov 08, 2019 at 12:17:49PM -0800, Eric Dumazet wrote:
> > > > On Fri, Nov 8, 2019 at 11:24 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > > >
> > > > > On Thu, Nov 07, 2019 at 11:37:37AM -0800, Eric Dumazet wrote:
> > > > > > We would like to use hlist_unhashed() from timer_pending(),
> > > > > > which runs without protection of a lock.
> > > > > >
> > > > > > Note that other callers might also want to use this variant.
> > > > > >
> > > > > > Instead of forcing a READ_ONCE() for all hlist_unhashed()
> > > > > > callers, add a new helper with an explicit _lockless suffix
> > > > > > in the name to better document what is going on.
> > > > > >
> > > > > > Also add various WRITE_ONCE() in __hlist_del(), hlist_add_head()
> > > > > > and hlist_add_before()/hlist_add_behind() to pair with
> > > > > > the READ_ONCE().
> > > > > >
> > > > > > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > > > > > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > > > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > >
> > > > > I have queued this, but if you prefer it go some other way:
> > > > >
> > > > > Acked-by: Paul E. McKenney <paulmck@kernel.org>
> > > > >
> > > > > But shouldn't the uses in include/linux/rculist.h also be converted
> > > > > into the patch below?  If so, I will squash the following into your
> > > > > patch.
> > > > >
> > > > >                                                 Thanx, Paul
> > > > >
> > > > > ------------------------------------------------------------------------
> > > >
> > > > Agreed, thanks for the addition of this Paul.
> > >
> > > Very good, squashed and pushed, thank you!
> > >
> > 
> > I have another KCSAN report of a bug that will force us to use
> > hlist_unhashed_lockless() from sk_unhashed()
> > 
> > (Meaning we also need to add some WRITE_ONCE() annotations to
> > include/linux/list_nulls.h )
> > 
> > BUG: KCSAN: data-race in inet_unhash / inet_unhash
> > 
> > write to 0xffff8880a69a0170 of 8 bytes by interrupt on cpu 1:
> >  __hlist_nulls_del include/linux/list_nulls.h:88 [inline]
> >  hlist_nulls_del_init_rcu include/linux/rculist_nulls.h:36 [inline]
> >  __sk_nulls_del_node_init_rcu include/net/sock.h:676 [inline]
> >  inet_unhash+0x38f/0x4a0 net/ipv4/inet_hashtables.c:612
> >  tcp_set_state+0xfa/0x3e0 net/ipv4/tcp.c:2249
> >  tcp_done+0x93/0x1e0 net/ipv4/tcp.c:3854
> >  tcp_write_err+0x7e/0xc0 net/ipv4/tcp_timer.c:56
> >  tcp_retransmit_timer+0x9b8/0x16d0 net/ipv4/tcp_timer.c:479
> >  tcp_write_timer_handler+0x42d/0x510 net/ipv4/tcp_timer.c:599
> >  tcp_write_timer+0xd1/0xf0 net/ipv4/tcp_timer.c:619
> >  call_timer_fn+0x5f/0x2f0 kernel/time/timer.c:1404
> >  expire_timers kernel/time/timer.c:1449 [inline]
> >  __run_timers kernel/time/timer.c:1773 [inline]
> >  __run_timers kernel/time/timer.c:1740 [inline]
> >  run_timer_softirq+0xc0c/0xcd0 kernel/time/timer.c:1786
> >  __do_softirq+0x115/0x33f kernel/softirq.c:292
> >  invoke_softirq kernel/softirq.c:373 [inline]
> >  irq_exit+0xbb/0xe0 kernel/softirq.c:413
> >  exiting_irq arch/x86/include/asm/apic.h:536 [inline]
> >  smp_apic_timer_interrupt+0xe6/0x280 arch/x86/kernel/apic/apic.c:1137
> >  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
> >  native_safe_halt+0xe/0x10 arch/x86/kernel/paravirt.c:71
> >  arch_cpu_idle+0x1f/0x30 arch/x86/kernel/process.c:571
> >  default_idle_call+0x1e/0x40 kernel/sched/idle.c:94
> >  cpuidle_idle_call kernel/sched/idle.c:154 [inline]
> >  do_idle+0x1af/0x280 kernel/sched/idle.c:263
> >  cpu_startup_entry+0x1b/0x20 kernel/sched/idle.c:355
> >  start_secondary+0x208/0x260 arch/x86/kernel/smpboot.c:264
> >  secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:241
> > 
> > read to 0xffff8880a69a0170 of 8 bytes by interrupt on cpu 0:
> >  sk_unhashed include/net/sock.h:607 [inline]
> >  inet_unhash+0x3d/0x4a0 net/ipv4/inet_hashtables.c:592
> >  tcp_set_state+0xfa/0x3e0 net/ipv4/tcp.c:2249
> >  tcp_done+0x93/0x1e0 net/ipv4/tcp.c:3854
> >  tcp_write_err+0x7e/0xc0 net/ipv4/tcp_timer.c:56
> >  tcp_retransmit_timer+0x9b8/0x16d0 net/ipv4/tcp_timer.c:479
> >  tcp_write_timer_handler+0x42d/0x510 net/ipv4/tcp_timer.c:599
> >  tcp_write_timer+0xd1/0xf0 net/ipv4/tcp_timer.c:619
> >  call_timer_fn+0x5f/0x2f0 kernel/time/timer.c:1404
> >  expire_timers kernel/time/timer.c:1449 [inline]
> >  __run_timers kernel/time/timer.c:1773 [inline]
> >  __run_timers kernel/time/timer.c:1740 [inline]
> >  run_timer_softirq+0xc0c/0xcd0 kernel/time/timer.c:1786
> >  __do_softirq+0x115/0x33f kernel/softirq.c:292
> >  invoke_softirq kernel/softirq.c:373 [inline]
> >  irq_exit+0xbb/0xe0 kernel/softirq.c:413
> >  exiting_irq arch/x86/include/asm/apic.h:536 [inline]
> >  smp_apic_timer_interrupt+0xe6/0x280 arch/x86/kernel/apic/apic.c:1137
> >  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
> >  native_safe_halt+0xe/0x10 arch/x86/kernel/paravirt.c:71
> >  arch_cpu_idle+0x1f/0x30 arch/x86/kernel/process.c:571
> >  default_idle_call+0x1e/0x40 kernel/sched/idle.c:94
> >  cpuidle_idle_call kernel/sched/idle.c:154 [inline]
> >  do_idle+0x1af/0x280 kernel/sched/idle.c:263
> >  cpu_startup_entry+0x1b/0x20 kernel/sched/idle.c:355
> >  rest_init+0xec/0xf6 init/main.c:452
> >  arch_call_rest_init+0x17/0x37
> >  start_kernel+0x838/0x85e init/main.c:786
> >  x86_64_start_reservations+0x29/0x2b arch/x86/kernel/head64.c:490
> >  x86_64_start_kernel+0x72/0x76 arch/x86/kernel/head64.c:471
> >  secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:241
> > 
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.4.0-rc6+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine,
> > BIOS Google 01/01/2011
> 
> Like this?

Hmmm...  Do you also need this?

						Thanx, Paul

------------------------------------------------------------------------

commit cf78c8772c9dc26a36c0e5eae1262cc396bbfb3f
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Sat Nov 9 10:45:47 2019 -0800

    rcu: Add a hlist_nulls_unhashed_lockless() function
    
    This commit adds an hlist_nulls_unhashed_lockless() to allow lockless
    checking for whether or note an hlist_nulls_node is hashed or not.
    While in the area, this commit also adds a docbook comment to the existing
    hlist_nulls_unhashed() function.
    
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index 1ecd356..fa6e847 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -56,11 +56,33 @@ static inline unsigned long get_nulls_value(const struct hlist_nulls_node *ptr)
 	return ((unsigned long)ptr) >> 1;
 }
 
+/**
+ * hlist_nulls_unhashed - Has node been removed and reinitialized?
+ * @h: Node to be checked
+ *
+ * Not that not all removal functions will leave a node in unhashed state.
+ * For example, hlist_del_init_rcu() leaves the node in unhashed state,
+ * but hlist_nulls_del() does not.
+ */
 static inline int hlist_nulls_unhashed(const struct hlist_nulls_node *h)
 {
 	return !h->pprev;
 }
 
+/**
+ * hlist_nulls_unhashed_lockless - Has node been removed and reinitialized?
+ * @h: Node to be checked
+ *
+ * Not that not all removal functions will leave a node in unhashed state.
+ * For example, hlist_del_init_rcu() leaves the node in unhashed state,
+ * but hlist_nulls_del() does not.  Unlike hlist_nulls_unhashed(), this
+ * function may be used locklessly.
+ */
+static inline int hlist_nulls_unhashed_lockless(const struct hlist_nulls_node *h)
+{
+	return !READ_ONCE(h->pprev);
+}
+
 static inline int hlist_nulls_empty(const struct hlist_nulls_head *h)
 {
 	return is_a_nulls(READ_ONCE(h->first));