On Mon, 12 Mar 2018 14:05:52 -0500 "Steven J. Hill" <steven.hill@xxxxxxxxxx> wrote: > Attempting to hotplug CPUs with CONFIG_VM_EVENT_COUNTERS enabled > can cause vmstat_update() to report a BUG due to preemption not > being disabled around smp_processor_id(). Discovered on Ubiquiti > EdgeRouter Pro with Cavium Octeon II processor. > > BUG: using smp_processor_id() in preemptible [00000000] code: > kworker/1:1/269 > caller is vmstat_update+0x50/0xa0 > CPU: 0 PID: 269 Comm: kworker/1:1 Not tainted > 4.16.0-rc4-Cavium-Octeon-00009-gf83bbd5-dirty #1 > Workqueue: mm_percpu_wq vmstat_update > Stack : 0000002600000026 0000000010009ce0 0000000000000000 0000000000000001 > 0000000000000000 0000000000000000 0000000000000005 8001180000000800 > 00000000000000bf 0000000000000000 00000000000000bf 766d737461745f75 > ffffffff83ad0000 0000000000000007 0000000000000000 0000000008000000 > 0000000000000000 ffffffff818d0000 0000000000000001 ffffffff81818a70 > 0000000000000000 0000000000000000 ffffffff8115bbb0 ffffffff818a0000 > 0000000000000005 ffffffff8144dc50 0000000000000000 0000000000000000 > 8000000088980000 8000000088983b30 0000000000000088 ffffffff813d3054 > 0000000000000000 ffffffff83ace622 00000000000000be 0000000000000000 > 00000000000000be ffffffff81121fb4 0000000000000000 0000000000000000 > ... > Call Trace: > [<ffffffff81121fb4>] show_stack+0x94/0x128 > [<ffffffff813d3054>] dump_stack+0xa4/0xe0 > [<ffffffff813fcfb8>] check_preemption_disabled+0x118/0x120 > [<ffffffff811eafd8>] vmstat_update+0x50/0xa0 > [<ffffffff8115b954>] process_one_work+0x144/0x348 > [<ffffffff8115bd00>] worker_thread+0x150/0x4b8 > [<ffffffff811622a0>] kthread+0x110/0x140 > [<ffffffff8111c304>] ret_from_kernel_thread+0x14/0x1c > > ... > > --- a/mm/vmstat.c > +++ b/mm/vmstat.c > @@ -1839,9 +1839,11 @@ static void vmstat_update(struct work_struct *w) > * to occur in the future. Keep on running the > * update worker thread. > */ > + preempt_disable(); > queue_delayed_work_on(smp_processor_id(), mm_percpu_wq, > this_cpu_ptr(&vmstat_work), > round_jiffies_relative(sysctl_stat_interval)); > + preempt_enable(); > } > } hm, I suspect this warning is a false-positive. vmstat_update() is called from a workqueue and so vmstat_update()'s execution is pinned to a particular CPU, so smp_processor_id()'s return will not be invalidated by a preemption-induced CPU switch. Unless the workqueue code changed more than I think it did ;) The patch is OK and will work, but I wonder if a better fix is to use raw_smp_processor_id() and to add a little comment explaining why it's needed and is safe.
