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Date:   Mon, 4 Nov 2019 08:26:52 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     syzbot <syzbot+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>, rcu@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, amir73il@gmail.com,
        darrick.wong@oracle.com, Johannes Weiner <hannes@cmpxchg.org>,
        jack@suse.cz, josef@toxicpanda.com,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        songliubraving@fb.com, syzkaller-bugs@googlegroups.com,
        willy@infradead.org
Subject: Re: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus
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 <CANpmjNMegMA6d5n1cV5JLmMbbe-3TkH5q3sNt9+E+wb8esjvoA@mail.gmail.com>
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On Mon, Nov 04, 2019 at 12:32:57PM +0100, Marco Elver wrote:
> +RCU folks
> 
> On Mon, 4 Nov 2019 at 12:30, syzbot
> <syzbot+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    05f22368 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=1296ff50e00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
> > dashboard link: https://syzkaller.appspot.com/bug?extid=99f4ddade3c22ab0cf23
> > 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+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus
> >
> > read to 0xffffffff85a7d440 of 8 bytes by task 7624 on cpu 0:
> >   rcu_read_unlock_special kernel/rcu/tree_plugin.h:615 [inline]
> >   __rcu_read_unlock+0x381/0x3c0 kernel/rcu/tree_plugin.h:383
> >   rcu_read_unlock include/linux/rcupdate.h:652 [inline]
> >   filemap_map_pages+0x5b3/0x990 mm/filemap.c:2687
> >   do_fault_around mm/memory.c:3450 [inline]
> >   do_read_fault mm/memory.c:3484 [inline]
> >   do_fault mm/memory.c:3618 [inline]
> >   handle_pte_fault mm/memory.c:3849 [inline]
> >   __handle_mm_fault+0x2554/0x2cb0 mm/memory.c:3973
> >   handle_mm_fault+0x21b/0x530 mm/memory.c:4010
> >   do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
> >   __do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
> >   do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
> >   page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202
> >
> > write to 0xffffffff85a7d440 of 8 bytes by task 7353 on cpu 1:
> >   sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
> >   sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
> >   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: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: rcu_gp wait_rcu_exp_gp
> > ==================================================================
> > Kernel panic - not syncing: panic_on_warn set ...
> > CPU: 1 PID: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: rcu_gp wait_rcu_exp_gp
> > Call Trace:
> >   __dump_stack lib/dump_stack.c:77 [inline]
> >   dump_stack+0xf5/0x159 lib/dump_stack.c:113
> >   panic+0x210/0x640 kernel/panic.c:221
> >   kcsan_report.cold+0xc/0x10 kernel/kcsan/report.c:302
> >   __kcsan_setup_watchpoint+0x32e/0x4a0 kernel/kcsan/core.c:411
> >   __tsan_write8 kernel/kcsan/kcsan.c:36 [inline]
> >   __tsan_write8+0x32/0x40 kernel/kcsan/kcsan.c:36
> >   sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
> >   sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
> >   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
> > Shutting down cpus with NMI
> > Kernel Offset: disabled
> > Rebooting in 86400 seconds..
> >
> >
> > ---
> > 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.

Again, good catch!  Does the patch shown below fix this?

							Thanx, Paul

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

commit c3c182aa501c13faccf5f2c168004a5c98a89d74
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Mon Nov 4 08:22:45 2019 -0800

    rcu: Use READ_ONCE() for ->expmask in rcu_read_unlock_special()
    
    The rcu_node structure's ->expmask field is updated only when holding the
    ->lock, but is also accessed locklessly.  This means that all ->expmask
    updates must use WRITE_ONCE() and all reads carried out without holding
    ->lock must use READ_ONCE().  This commit therefore changes the lockless
    ->expmask read in rcu_read_unlock_special() to use READ_ONCE().
    
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6d54fd4..9a1b4b2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -601,7 +601,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		struct rcu_node *rnp = rdp->mynode;
 
 		exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
-		      (rdp->grpmask & rnp->expmask) ||
+		      (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
 		      tick_nohz_full_cpu(rdp->cpu);
 		// Need to defer quiescent state until everything is enabled.
 		if (irqs_were_disabled && use_softirq &&