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Date:   Mon, 7 Oct 2019 18:47:43 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     syzbot <syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com>,
        josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org,
        a@unstable.cc, b.a.t.m.a.n@lists.open-mesh.org,
        davem@davemloft.net, LKML <linux-kernel@vger.kernel.org>,
        mareklindner@neomailbox.ch, netdev@vger.kernel.org,
        sw@simonwunderlich.de, syzkaller-bugs@googlegroups.com
Subject: Re: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult
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 <CANpmjNNmSOagbTpffHr4=Yedckx9Rm2NuGqC9UqE+AOz5f1-ZQ@mail.gmail.com>
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On Mon, Oct 07, 2019 at 12:04:16PM +0200, Marco Elver wrote:
> +RCU maintainers
> This might be a data-race in RCU itself.
> 
> On Mon, 7 Oct 2019 at 12:01, syzbot
> <syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    b4bd9343 x86, kcsan: Enable KCSAN for x86
> > git tree:       https://github.com/google/ktsan.git kcsan
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11edb20d600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
> > dashboard link: https://syzkaller.appspot.com/bug?extid=134336b86f728d6e55a0
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult
> >
> > write to 0xffffffff85a7f140 of 8 bytes by task 7 on cpu 0:
> >   rcu_report_exp_cpu_mult+0x4f/0xa0 kernel/rcu/tree_exp.h:244
> >   rcu_report_exp_rdp+0x6c/0x90 kernel/rcu/tree_exp.h:254
> >   rcu_preempt_deferred_qs_irqrestore+0x3bb/0x580 kernel/rcu/tree_plugin.h:475
> >   rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659
> >   __rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394
> >   rcu_read_unlock include/linux/rcupdate.h:645 [inline]
> >   batadv_nc_purge_orig_hash net/batman-adv/network-coding.c:411 [inline]
> >   batadv_nc_worker+0x13a/0x390 net/batman-adv/network-coding.c:718
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > read to 0xffffffff85a7f140 of 8 bytes by task 7251 on cpu 1:
> >   _find_next_bit lib/find_bit.c:39 [inline]
> >   find_next_bit+0x57/0xe0 lib/find_bit.c:70
> >   sync_rcu_exp_select_node_cpus+0x28e/0x510 kernel/rcu/tree_exp.h:375
> >   sync_rcu_exp_select_cpus+0x30c/0x590 kernel/rcu/tree_exp.h:439
> >   rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:575 [inline]
> >   wait_rcu_exp_gp+0x25/0x40 kernel/rcu/tree_exp.h:589
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 PID: 7251 Comm: kworker/1:4 Not tainted 5.3.0+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: rcu_gp wait_rcu_exp_gp
> > ==================================================================
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

And yes, ->expmask is accessed locklessly in a few places without
READ_ONCE().  So does the following (untested) patch help?

							Thanx, Paul

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

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index d632cd0..bcd2a79 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -134,7 +134,7 @@ static void __maybe_unused sync_exp_reset_tree(void)
 	rcu_for_each_node_breadth_first(rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		WARN_ON_ONCE(rnp->expmask);
-		rnp->expmask = rnp->expmaskinit;
+		WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
 }
@@ -211,7 +211,7 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 		rnp = rnp->parent;
 		raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
 		WARN_ON_ONCE(!(rnp->expmask & mask));
-		rnp->expmask &= ~mask;
+		WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 	}
 }
 
@@ -241,7 +241,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		return;
 	}
-	rnp->expmask &= ~mask;
+	WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
 	__rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
 }
 
@@ -332,6 +332,7 @@ static bool exp_funnel_lock(unsigned long s)
 static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 {
 	int cpu;
+	unsigned long expmask_snap;
 	unsigned long flags;
 	unsigned long mask_ofl_test;
 	unsigned long mask_ofl_ipi;
@@ -369,10 +370,11 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 	 */
 	if (rcu_preempt_has_tasks(rnp))
 		rnp->exp_tasks = rnp->blkd_tasks.next;
+	expmask_snap = rnp->expmask;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
 	/* IPI the remaining CPUs for expedited quiescent state. */
-	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
+	for_each_leaf_node_cpu_mask(rnp, cpu, expmask_snap) {
 		unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
@@ -491,7 +493,7 @@ static void synchronize_sched_expedited_wait(void)
 				struct rcu_data *rdp;
 
 				mask = leaf_node_cpu_bit(rnp, cpu);
-				if (!(rnp->expmask & mask))
+				if (!(READ_ONCE(rnp->expmask) & mask))
 					continue;
 				ndetected++;
 				rdp = per_cpu_ptr(&rcu_data, cpu);
@@ -503,7 +505,8 @@ static void synchronize_sched_expedited_wait(void)
 		}
 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 			jiffies - jiffies_start, rcu_state.expedited_sequence,
-			rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
+			READ_ONCE(rnp_root->expmask),
+			".T"[!!rnp_root->exp_tasks]);
 		if (ndetected) {
 			pr_err("blocking rcu_node structures:");
 			rcu_for_each_node_breadth_first(rnp) {
@@ -513,7 +516,7 @@ static void synchronize_sched_expedited_wait(void)
 					continue;
 				pr_cont(" l=%u:%d-%d:%#lx/%c",
 					rnp->level, rnp->grplo, rnp->grphi,
-					rnp->expmask,
+					READ_ONCE(rnp->expmask),
 					".T"[!!rnp->exp_tasks]);
 			}
 			pr_cont("\n");
@@ -521,7 +524,7 @@ static void synchronize_sched_expedited_wait(void)
 		rcu_for_each_leaf_node(rnp) {
 			for_each_leaf_node_possible_cpu(rnp, cpu) {
 				mask = leaf_node_cpu_bit(rnp, cpu);
-				if (!(rnp->expmask & mask))
+				if (!(READ_ONCE(rnp->expmask) & mask))
 					continue;
 				dump_cpu_task(cpu);
 			}