Messages in this thread | | | From | Dmitry Vyukov <> | Date | Sun, 5 Mar 2017 11:50:39 +0100 | Subject | Re: rcu: WARNING in rcu_seq_end |
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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)
Mailed a signed patch: https://groups.google.com/d/msg/syzkaller/XzUXuAzKkCw/5054wU9MEAAJ
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