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SubjectRe: rcu: WARNING in rcu_seq_end
On Sun, Mar 05, 2017 at 11:50:39AM +0100, Dmitry Vyukov wrote:
> On Sat, Mar 4, 2017 at 9:40 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Sat, Mar 04, 2017 at 05:01:19PM +0100, Dmitry Vyukov wrote:
> >> Hello,
> >>
> >> Paul, you wanted bugs in rcu.
> >
> > Well, whether I want them or not, I must deal with them. ;-)
> >
> >> I've got this WARNING while running syzkaller fuzzer on
> >> 86292b33d4b79ee03e2f43ea0381ef85f077c760:
> >>
> >> ------------[ cut here ]------------
> >> WARNING: CPU: 0 PID: 4832 at kernel/rcu/tree.c:3533
> >> rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
> >> Kernel panic - not syncing: panic_on_warn set ...
> >> CPU: 0 PID: 4832 Comm: kworker/0:3 Not tainted 4.10.0+ #276
> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >> Workqueue: events wait_rcu_exp_gp
> >> Call Trace:
> >> __dump_stack lib/dump_stack.c:15 [inline]
> >> dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
> >> panic+0x1fb/0x412 kernel/panic.c:179
> >> __warn+0x1c4/0x1e0 kernel/panic.c:540
> >> warn_slowpath_null+0x2c/0x40 kernel/panic.c:583
> >> rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
> >> rcu_exp_gp_seq_end kernel/rcu/tree_exp.h:36 [inline]
> >> rcu_exp_wait_wake+0x8a9/0x1330 kernel/rcu/tree_exp.h:517
> >> rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:559 [inline]
> >> wait_rcu_exp_gp+0x83/0xc0 kernel/rcu/tree_exp.h:570
> >> process_one_work+0xc06/0x1c20 kernel/workqueue.c:2096
> >> worker_thread+0x223/0x19c0 kernel/workqueue.c:2230
> >> kthread+0x326/0x3f0 kernel/kthread.c:227
> >> ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430
> >> Dumping ftrace buffer:
> >> (ftrace buffer empty)
> >> Kernel Offset: disabled
> >> Rebooting in 86400 seconds..
> >>
> >>
> >> Not reproducible. But looking at the code, shouldn't it be:
> >>
> >> static void rcu_seq_end(unsigned long *sp)
> >> {
> >> smp_mb(); /* Ensure update-side operation before counter increment. */
> >> + WARN_ON_ONCE(!(*sp & 0x1));
> >> WRITE_ONCE(*sp, *sp + 1);
> >> - WARN_ON_ONCE(*sp & 0x1);
> >> }
> >>
> >> ?
> >>
> >> Otherwise wait_event in _synchronize_rcu_expedited can return as soon
> >> as WRITE_ONCE(*sp, *sp + 1) finishes. As far as I understand this
> >> consequently can allow start of next grace periods. Which in turn can
> >> make the warning fire. Am I missing something?
> >>
> >> I don't see any other bad consequences of this. The rest of
> >> rcu_exp_wait_wake can proceed when _synchronize_rcu_expedited has
> >> returned and destroyed work on stack and next period has started and
> >> ended, but it seems OK.
> >
> > I believe that this is a heygood change, but I don't see how it will
> > help in this case. BTW, may I have your Signed-off-by?
> >
> > The reason I don't believe that it will help is that the
> > rcu_exp_gp_seq_end() function is called from a workqueue handler that
> > is invoked holding ->exp_mutex, and this mutex is not released until
> > after the handler invokes rcu_seq_end() and then wakes up the task that
> > scheduled the workqueue handler. So the ordering above should not matter
> > (but I agree that your ordering is cleaner.
> >
> > That said, it looks like I am missing some memory barriers, please
> > see the following patch.
> >
> > But what architecture did you see this on?
>
>
> This is just x86.
>
> You seem to assume that wait_event() waits for the wakeup. It does not
> work this way. It can return as soon as the condition becomes true
> without ever waiting:
>
> 305 #define wait_event(wq, condition) \
> 306 do { \
> 307 might_sleep(); \
> 308 if (condition) \
> 309 break; \
> 310 __wait_event(wq, condition); \
> 311 } while (0)

Agreed, hence my patch in the previous email. I guess I knew that, but
on the day I wrote that code, my fingers didn't. Or somew similar lame
excuse. ;-)

> Mailed a signed patch:
> https://groups.google.com/d/msg/syzkaller/XzUXuAzKkCw/5054wU9MEAAJ

This is the patch you also sent by email, that moves the WARN_ON_ONCE(),
thank you!

Thanx, Paul
>

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