Messages in this thread | | | Subject | Re: [PATCH -next] nbd: Fix hungtask when nbd_config_put | From | yebin <> | Date | Wed, 20 Oct 2021 22:23:45 +0800 |
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On 2021/10/20 22:04, Josef Bacik wrote: > On Wed, Oct 20, 2021 at 05:48:30PM +0800, Ye Bin wrote: >> I got follow issue: >> [ 247.381177] INFO: task kworker/u10:0:47 blocked for more than 120 seconds. >> [ 247.382644] Not tainted 4.19.90-dirty #140 >> [ 247.383502] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >> [ 247.385027] Call Trace: >> [ 247.388384] schedule+0xb8/0x3c0 >> [ 247.388966] schedule_timeout+0x2b4/0x380 >> [ 247.392815] wait_for_completion+0x367/0x510 >> [ 247.397713] flush_workqueue+0x32b/0x1340 >> [ 247.402700] drain_workqueue+0xda/0x3c0 >> [ 247.403442] destroy_workqueue+0x7b/0x690 >> [ 247.405014] nbd_config_put.cold+0x2f9/0x5b6 >> [ 247.405823] recv_work+0x1fd/0x2b0 >> [ 247.406485] process_one_work+0x70b/0x1610 >> [ 247.407262] worker_thread+0x5a9/0x1060 >> [ 247.408699] kthread+0x35e/0x430 >> [ 247.410918] ret_from_fork+0x1f/0x30 >> >> We can reprodeuce issue as follows: >> 1. Inject memory fault in nbd_start_device >> @@ -1244,10 +1248,18 @@ static int nbd_start_device(struct nbd_device *nbd) >> nbd_dev_dbg_init(nbd); >> for (i = 0; i < num_connections; i++) { >> struct recv_thread_args *args; >> - >> - args = kzalloc(sizeof(*args), GFP_KERNEL); >> + >> + if (i == 1) { >> + args = NULL; >> + printk("%s: inject malloc error\n", __func__); >> + } >> + else >> + args = kzalloc(sizeof(*args), GFP_KERNEL); >> 2. Inject delay in recv_work >> @@ -757,6 +760,8 @@ static void recv_work(struct work_struct *work) >> >> blk_mq_complete_request(blk_mq_rq_from_pdu(cmd)); >> } >> + printk("%s: comm=%s pid=%d\n", __func__, current->comm, current->pid); >> + mdelay(5 * 1000); >> nbd_config_put(nbd); >> atomic_dec(&config->recv_threads); >> wake_up(&config->recv_wq); >> 3. Create nbd server >> nbd-server 8000 /tmp/disk >> 4. Create nbd client >> nbd-client localhost 8000 /dev/nbd1 >> Then will trigger above issue. >> >> Reason is when add delay in recv_work, lead to relase the last reference >> of 'nbd->config_refs'. nbd_config_put will call flush_workqueue to make >> all work finish. Obviously, it will lead to deadloop. >> To solve this issue, we must ensure 'recv_work' all exit before release >> the last 'nbd->config_refs' reference count. >> >> Signed-off-by: Ye Bin <yebin10@huawei.com> > This isn't the way to fix this. The crux of the problem is the fact that we > destroy the recv_workq at config time anyway, since it is tied to the nbd > device. > > Fix this instead by getting rid of that, and have the nbd device teardown the > recv_workq when we destory the nbd device. Alloc it on device allocation and > stop alloc'ing it with the config. That way we don't have this weird thing > where we need recv threads to drop their config before the last thing. > > And while you're at it would you fix recv_work() so it doesn't modify config > after it drops it's reference? Thanks, > > Josef > . Thank you for your reply, I'll make another patch according to your suggestion.
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