Re: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus

[Marco Elver <elver@google.com>]

On Mon, 4 Nov 2019 at 17:26, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> 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?

Assuming writers will follow in another patch:

Acked-by: Marco Elver <elver@google.com>

Thanks,
-- Marco


>                                                         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 &&