On Mon, Sep 18, 2017 at 09:04:56PM -0700, Paul E. McKenney wrote: > On Tue, Sep 19, 2017 at 11:48:22AM +0900, Byungchul Park wrote: > > On Mon, Sep 18, 2017 at 07:33:29PM -0700, Paul E. McKenney wrote: > > > > > Hello Paul and Steven, > > > > >
So I think this is another false positive, and the reason is we use st->done for multiple purposes. > > > > > This is saying: > > > > > > > > > > Thread A > > > > > -------- > > > > > takedown_cpu() > > > > > irq_lock_sparse() > > > > > wait_for_completion(&st->done) // Wait for completion of B Thread A wait for the idle task on the outgoing to set the st->state to CPUHP_AP_IDLE_DEAD(i.e. the corresponding complete() is the one in cpuhp_complete_idle_dead()), and it happens when we try to _offline_ a cpu. > > > > > irq_unlock_sparse() > > > > > > > > > > Thread B > > > > > -------- > > > > > cpuhp_invoke_callback() > > > > > irq_lock_sparse() // Wait for A to irq_unlock_sparse() irq_affinity_online_cpu() is called here, so it happens when we try to _online_ a cpu. > > > > > (on the way going to complete(&st->done)) and we are going to complete(&st->done) in a hotplug thread context to indicate the hotplug thread has finished its job(i.e. this complete() is the one in cpuhp_thread_fun()). So even though the &st->done are the same instance, the deadlock could not happen, I think, as we could not up/down a same cpu at the same time? If I'm not missing something subtle. To fix this we can either 1) have dedicated completion instances for different wait purposes in cpuhp_cpu_state. or 2) extend crossrelease to have the "subclass" concept, so that callsite of complete() and wait_for_completion() for the same completion instance but with different purposes could be differed by lockdep. Thoughts? Regards, Boqun > > > > > > > > > > So, lockdep consider this as a deadlock. > > > > > Is it possible to happen? > > > > > > > > In addition, if it's impossible, then we should fix lock class > > > > assignments so that the locks actually have different classes. > > > > > > Interesting, and thank you for the analysis! > > > > > > The strange thing is that the way you describe it, this would be a > > > deterministic deadlock. Yet CPU hotplug operations complete just fine > > > in my tests. What am I missing here? > > > > Hi, :) > > > > Lockdep basically reports either (1) an actual deadlock happened at the > > time or (2) a deadlock possibility, even w/o LOCKDEP_CROSSRELEASE. > > > > Both are useful. But LOCKDEP_CROSSRELEASE can only do the latter. IOW, > > the deadlock would actually happen _only_ when the two threads(A and B) > > run simultaniously. > > > > In your case, those two threads might run at different timings. So it's > > not an actual deadlock, but still has a possibility for the problem to > > happen later. > > Fair enough, if the wakeup always happened first, deadlock might well > be avoided. If the sleep happened first, I suspect deadlock would > be deterministic in this case. > > Thanx, Paul > > > > > > Thanks, > > > > > Byungchul > > > > > > > > > > > [ 35.313943] > > > > > > [ 35.313943] 3 locks held by torture_onoff/766: > > > > > > [ 35.313943] #0: (cpu_add_remove_lock){+.+.}, at: > > > > > > [<ffffffffb9060be2>] do_cpu_down+0x22/0x50 > > > > > > [ 35.313943] #1: (cpu_hotplug_lock.rw_sem){++++}, at: > > > > > > [<ffffffffb90acc41>] percpu_down_write+0x21/0xf0 > > > > > > [ 35.313943] #2: (sparse_irq_lock){+.+.}, at: > > > > > > [<ffffffffb90c5e42>] irq_lock_sparse+0x12/0x20 > > > > > > [ 35.313943] > > > > > > [ 35.313943] stack backtrace: > > > > > > [ 35.313943] CPU: 7 PID: 766 Comm: torture_onoff Not tainted > > > > > > 4.13.0-rc4+ #1 > > > > > > [ 35.313943] Hardware name: QEMU Standard PC (i440FX + PIIX, > > > > > > 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014 > > > > > > [ 35.313943] Call Trace: > > > > > > [ 35.313943] dump_stack+0x67/0x97 > > > > > > [ 35.313943] print_circular_bug+0x21d/0x330 > > > > > > [ 35.313943] ? add_lock_to_list.isra.31+0xc0/0xc0 > > > > > > [ 35.313943] check_prev_add+0x401/0x800 > > > > > > [ 35.313943] ? wake_up_q+0x70/0x70 > > > > > > [ 35.313943] __lock_acquire+0x1100/0x11a0 > > > > > > [ 35.313943] ? __lock_acquire+0x1100/0x11a0 > > > > > > [ 35.313943] ? add_lock_to_list.isra.31+0xc0/0xc0 > > > > > > [ 35.313943] lock_acquire+0x9e/0x1e0 > > > > > > [ 35.313943] ? takedown_cpu+0x86/0xf0 > > > > > > [ 35.313943] wait_for_completion+0x36/0x130 > > > > > > [ 35.313943] ? takedown_cpu+0x86/0xf0 > > > > > > [ 35.313943] ? stop_machine_cpuslocked+0xb9/0xd0 > > > > > > [ 35.313943] ? cpuhp_invoke_callback+0x8b0/0x8b0 > > > > > > [ 35.313943] ? cpuhp_complete_idle_dead+0x10/0x10 > > > > > > [ 35.313943] takedown_cpu+0x86/0xf0 > > > > > > [ 35.313943] cpuhp_invoke_callback+0xa7/0x8b0 > > > > > > [ 35.313943] cpuhp_down_callbacks+0x3d/0x80 > > > > > > [ 35.313943] _cpu_down+0xbb/0xf0 > > > > > > [ 35.313943] do_cpu_down+0x39/0x50 > > > > > > [ 35.313943] cpu_down+0xb/0x10 > > > > > > [ 35.313943] torture_offline+0x75/0x140 > > > > > > [ 35.313943] torture_onoff+0x102/0x1e0 > > > > > > [ 35.313943] kthread+0x142/0x180 > > > > > > [ 35.313943] ? torture_kthread_stopping+0x70/0x70 > > > > > > [ 35.313943] ? kthread_create_on_node+0x40/0x40 > > > > > > [ 35.313943] ret_from_fork+0x27/0x40 > > > > > > >
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