Hi John, On 6/3/20 4:59 AM, John Garry wrote: > On 02/06/2020 07:17, Dongli Zhang wrote: >> When scheduler is set, we hit below page fault when we offline cpu. >> >> [ 1061.007725] BUG: kernel NULL pointer dereference, address: 0000000000000040 >> [ 1061.008710] #PF: supervisor read access in kernel mode >> [ 1061.009492] #PF: error_code(0x0000) - not-present page >> [ 1061.010241] PGD 0 P4D 0 >> [ 1061.010614] Oops: 0000 [#1] SMP PTI >> [ 1061.011130] CPU: 0 PID: 122 Comm: kworker/0:1H Not tainted 5.7.0-rc7+ #2' >> ... ... >> [ 1061.013760] Workqueue: kblockd blk_mq_run_work_fn >> [ 1061.014446] RIP: 0010:blk_mq_put_tag+0xf/0x30 >> ... ... >> [ 1061.017726] RSP: 0018:ffffa5c18037fc70 EFLAGS: 00010287 >> [ 1061.018475] RAX: 0000000000000000 RBX: ffffa5c18037fcf0 RCX: 0000000000000004 >> [ 1061.019507] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff911535dc1180 >> ... ... >> [ 1061.028454] Call Trace: >> [ 1061.029307] blk_mq_get_tag+0x26e/0x280 >> [ 1061.029866] ? wait_woken+0x80/0x80 >> [ 1061.030378] blk_mq_get_driver_tag+0x99/0x110 >> [ 1061.031009] blk_mq_dispatch_rq_list+0x107/0x5e0 >> [ 1061.031672] ? elv_rb_del+0x1a/0x30 >> [ 1061.032178] blk_mq_do_dispatch_sched+0xe2/0x130 >> [ 1061.032844] __blk_mq_sched_dispatch_requests+0xcc/0x150 >> [ 1061.033638] blk_mq_sched_dispatch_requests+0x2b/0x50 >> [ 1061.034239] __blk_mq_run_hw_queue+0x75/0x110 >> [ 1061.034867] process_one_work+0x15c/0x370 >> [ 1061.035450] worker_thread+0x44/0x3d0 >> [ 1061.035980] kthread+0xf3/0x130 >> [ 1061.036440] ? max_active_store+0x80/0x80 >> [ 1061.037018] ? kthread_bind+0x10/0x10 >> [ 1061.037554] ret_from_fork+0x35/0x40 >> [ 1061.038073] Modules linked in: >> [ 1061.038543] CR2: 0000000000000040 >> [ 1061.038962] ---[ end trace d20e1df7d028e69f ]--- >> >> This is because blk_mq_get_driver_tag() would be used to allocate tag once >> scheduler (e.g., mq-deadline) is set. > > I tried mq-deadline and I did not see this. Anyway else special or specific > about your test? > I think you just hit the issue as mentioned in another thread. To reproduce the issue reproduce to hit the condition that: 1. blk_mq_hctx_notify_offline() BLK_MQ_S_INACTIVE with the barrier ... ... while ... 2. blk_mq_get_tag() gets the tag but BLK_MQ_S_INACTIVE is already set. Therefore, it would put the tag to release it. Dongli Zhang
