[PATCH RFC] nvmet-rdma: use a private workqueue for delete

[bvanassche@acm.org (Bart Van Assche)]

On Thu, 2018-09-27@11:00 -0700, Sagi Grimberg wrote:
> Queue deletion is done asynchronous when the last reference on
> the queue is dropped. Thus, in order to make sure we don't over
> allocate under a connect/disconnect storm, we let queue deletion
> complete before making forward progress.
> 
> However, given that we flush the system_wq from rdma_cm context
> which runs from a workqueue context, we can have a circular
> locking complaint [1]. Fix that by using a private workqueue for
> queue deletion.

Hi Sagi,

Thanks for this patch. With this patch applied the warning I reported
earlier disappears but a new warning appeared:

======================================================
WARNING: possible circular locking dependency detected
4.19.0-rc5-dbg+ #8 Not tainted
------------------------------------------------------
kworker/u24:7/977 is trying to acquire lock:
000000003dc8471f (&id_priv->handler_mutex){+.+.}, at: rdma_destroy_id+0x6f/0x440 [rdma_cm]

but task is already holding lock:
0000000086ca5cb1 ((work_completion)(&queue->release_work)){+.+.}, at: process_one_work+0x3ed/0xa20

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 ((work_completion)(&queue->release_work)){+.+.}:
       process_one_work+0x474/0xa20
       worker_thread+0x63/0x5a0
       kthread+0x1cf/0x1f0
       ret_from_fork+0x24/0x30

-> #2 ((wq_completion)"nvmet-rdma-delete-wq"){+.+.}:
       flush_workqueue+0xf3/0x970
       nvmet_rdma_cm_handler+0x1319/0x170f [nvmet_rdma]
       cma_ib_req_handler+0x72f/0xf90 [rdma_cm]
       cm_process_work+0x2e/0x110 [ib_cm]
       cm_req_handler+0x135b/0x1c30 [ib_cm]
       cm_work_handler+0x2b7/0x38cd [ib_cm]
       process_one_work+0x4ae/0xa20
       worker_thread+0x63/0x5a0
       kthread+0x1cf/0x1f0
       ret_from_fork+0x24/0x30

-> #1 (&id_priv->handler_mutex/1){+.+.}:
       __mutex_lock+0xfe/0xbe0
       mutex_lock_nested+0x1b/0x20
       cma_ib_req_handler+0x6aa/0xf90 [rdma_cm]
       cm_process_work+0x2e/0x110 [ib_cm]
       cm_req_handler+0x135b/0x1c30 [ib_cm]
       cm_work_handler+0x2b7/0x38cd [ib_cm]
       process_one_work+0x4ae/0xa20
       worker_thread+0x63/0x5a0
       kthread+0x1cf/0x1f0
       ret_from_fork+0x24/0x30

-> #0 (&id_priv->handler_mutex){+.+.}:
       lock_acquire+0xc5/0x200
       __mutex_lock+0xfe/0xbe0
       mutex_lock_nested+0x1b/0x20
       rdma_destroy_id+0x6f/0x440 [rdma_cm]
       nvmet_rdma_release_queue_work+0x8e/0x1b0 [nvmet_rdma]
       process_one_work+0x4ae/0xa20
       worker_thread+0x63/0x5a0
       kthread+0x1cf/0x1f0
       ret_from_fork+0x24/0x30

other info that might help us debug this:

Chain exists of:
  &id_priv->handler_mutex --> (wq_completion)"nvmet-rdma-delete-wq" --> (work_completion)(&queue->release_work)

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock((work_completion)(&queue->release_work));
                               lock((wq_completion)"nvmet-rdma-delete-wq");
                               lock((work_completion)(&queue->release_work));
  lock(&id_priv->handler_mutex);

 *** DEADLOCK ***

2 locks held by kworker/u24:7/977:
 #0: 00000000148450ab ((wq_completion)"nvmet-rdma-delete-wq"){+.+.}, at: process_one_work+0x3ed/0xa20
 #1: 0000000086ca5cb1 ((work_completion)(&queue->release_work)){+.+.}, at: process_one_work+0x3ed/0xa20

stack backtrace:
CPU: 1 PID: 977 Comm: kworker/u24:7 Not tainted 4.19.0-rc5-dbg+ #8
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Workqueue: nvmet-rdma-delete-wq nvmet_rdma_release_queue_work [nvmet_rdma]
Call Trace:
 dump_stack+0x86/0xc5
 print_circular_bug.isra.32+0x20a/0x218
 __lock_acquire+0x1a54/0x1b20
 lock_acquire+0xc5/0x200
 __mutex_lock+0xfe/0xbe0
 mutex_lock_nested+0x1b/0x20
 rdma_destroy_id+0x6f/0x440 [rdma_cm]
 nvmet_rdma_release_queue_work+0x8e/0x1b0 [nvmet_rdma]
 process_one_work+0x4ae/0xa20
 worker_thread+0x63/0x5a0
 kthread+0x1cf/0x1f0
 ret_from_fork+0x24/0x30