Messages in this thread | | | Date | Mon, 6 Mar 2017 15:08:00 -0800 | From | "Paul E. McKenney" <> | Subject | Re: rcu: WARNING in rcu_seq_end |
| |
On Mon, Mar 06, 2017 at 11:11:23AM +0100, Dmitry Vyukov wrote: > On Mon, Mar 6, 2017 at 11:07 AM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > On Mon, Mar 06, 2017 at 10:24:24AM +0100, Dmitry Vyukov wrote: > >> On Sun, Mar 5, 2017 at 7:47 PM, Paul E. McKenney > >> <paulmck@linux.vnet.ibm.com> wrote: > >> > 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 > >> > >> Ah, you meant to synchronize rcu_seq_end with rcu_seq_done? > > > > No, there is a mutex release and acquisition that do the synchronization, > > but only if the wakeup has appropriate barriers. The issue is that > > part of the mutex's critical section executes in a workqueue, possibly > > on some other CPU. > > What is that mutex? And what locks/unlocks provide synchronization? I > see that one uses exp_mutex and another -- exp_wake_mutex.
Both of them.
->exp_mutex is acquired by the task requesting the grace period, and the counter's first increment is done by that task under that mutex. This task then schedules a workqueue, which drives forward the grace period. Upon grace-period completion, the workqueue handler does the second increment (the one that your patch addressed). The workqueue handler then acquires ->exp_wake_mutex and wakes the task that holds ->exp_mutex (along with all other tasks waiting for this grace period), and that task releases ->exp_mutex, which allows the next grace period to start (and the first increment for that next grace period to be carried out under that lock). The workqueue handler releases ->exp_wake_mutex after finishing its wakeups.
Does that make sense?
Thanx, Paul
| |