Lee/Chris: Please review!
> iscsi suffers from a deadlock in case a management command submitted via
> the netlink socket sleeps on an allocation while holding the
> rx_queue_mutex, if that allocation causes a memory reclaim that
> writebacks to a failed iscsi device. Then, the recovery procedure can
> never make progress to recover the failed disk or abort outstanding IO
> operations to complete the reclaim (since rx_queue_mutex is locked),
> thus locking the system.
>
> Nevertheless, just marking all allocations under rx_queue_mutex as
> GFP_NOIO (or locking the userspace process with something like
> PF_MEMALLOC_NOIO) is not enough, since the iscsi command code relies on
> other subsystems that try to grab locked mutexes, whose threads are
> GFP_IO, leading to the same deadlock. One instance where this situation
> can be observed is in the backtraces below, stitched from multiple bugs
> reports, involving the kobj uevent sent when a session is created.
>
> The root of the problem is not the fact that iscsi does GFP_IO
> allocations, that is acceptable. The actual problem is that
> rx_queue_mutex has a very large granularity, covering every unrelated
> netlink command execution at the same time as the error recovery path.
>
> The proposed fix leverages the recently added mechanism to stop failed
> connections from the kernel, by enabling it to execute even though a
> management command from the netlink socket is being run (rx_queue_mutex
> is held), provided that the command is known to be safe. It splits the
> rx_queue_mutex in two mutexes, one protecting from concurrent command
> execution from the netlink socket, and one protecting stop_conn from
> racing with other connection management operations that might conflict
> with it.
>
> It is not very pretty, but it is the simplest way to resolve the
> deadlock. I considered making it a lock per connection, but some
> external mutex would still be needed to deal with iscsi_if_destroy_conn.
>
> The patch was tested by forcing a memory shrinker (unrelated, but used
> bufio/dm-verity) to reclaim ISCSI pages every time
> ISCSI_UEVENT_CREATE_SESSION happens, which is reasonable to simulate
> reclaims that might happen with GFP_KERNEL on that path. Then, a faulty
> hung target causes a connection to fail during intensive IO, at the same
> time a new session is added by iscsid.
>
> The following stacktraces are stiches from several bug reports, showing
> a case where the deadlock can happen.
>
> iSCSI-write
> holding: rx_queue_mutex
> waiting: uevent_sock_mutex
>
> kobject_uevent_env+0x1bd/0x419
> kobject_uevent+0xb/0xd
> device_add+0x48a/0x678
> scsi_add_host_with_dma+0xc5/0x22d
> iscsi_host_add+0x53/0x55
> iscsi_sw_tcp_session_create+0xa6/0x129
> iscsi_if_rx+0x100/0x1247
> netlink_unicast+0x213/0x4f0
> netlink_sendmsg+0x230/0x3c0
>
> iscsi_fail iscsi_conn_failure
> waiting: rx_queue_mutex
>
> schedule_preempt_disabled+0x325/0x734
> __mutex_lock_slowpath+0x18b/0x230
> mutex_lock+0x22/0x40
> iscsi_conn_failure+0x42/0x149
> worker_thread+0x24a/0xbc0
>
> EventManager_
> holding: uevent_sock_mutex
> waiting: dm_bufio_client->lock
>
> dm_bufio_lock+0xe/0x10
> shrink+0x34/0xf7
> shrink_slab+0x177/0x5d0
> do_try_to_free_pages+0x129/0x470
> try_to_free_mem_cgroup_pages+0x14f/0x210
> memcg_kmem_newpage_charge+0xa6d/0x13b0
> __alloc_pages_nodemask+0x4a3/0x1a70
> fallback_alloc+0x1b2/0x36c
> __kmalloc_node_track_caller+0xb9/0x10d0
> __alloc_skb+0x83/0x2f0
> kobject_uevent_env+0x26b/0x419
> dm_kobject_uevent+0x70/0x79
> dev_suspend+0x1a9/0x1e7
> ctl_ioctl+0x3e9/0x411
> dm_ctl_ioctl+0x13/0x17
> do_vfs_ioctl+0xb3/0x460
> SyS_ioctl+0x5e/0x90
>
> MemcgReclaimerD"
> holding: dm_bufio_client->lock
> waiting: stuck io to finish (needs iscsi_fail thread to progress)
>
> schedule at ffffffffbd603618
> io_schedule at ffffffffbd603ba4
> do_io_schedule at ffffffffbdaf0d94
> __wait_on_bit at ffffffffbd6008a6
> out_of_line_wait_on_bit at ffffffffbd600960
> wait_on_bit.constprop.10 at ffffffffbdaf0f17
> __make_buffer_clean at ffffffffbdaf18ba
> __cleanup_old_buffer at ffffffffbdaf192f
> shrink at ffffffffbdaf19fd
> do_shrink_slab at ffffffffbd6ec000
> shrink_slab at ffffffffbd6ec24a
> do_try_to_free_pages at ffffffffbd6eda09
> try_to_free_mem_cgroup_pages at ffffffffbd6ede7e
> mem_cgroup_resize_limit at ffffffffbd7024c0
> mem_cgroup_write at ffffffffbd703149
> cgroup_file_write at ffffffffbd6d9c6e
> sys_write at ffffffffbd6662ea
> system_call_fastpath at ffffffffbdbc34a2
>
> Reported-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
> ---
> drivers/scsi/scsi_transport_iscsi.c | 67 +++++++++++++++++++++--------
> 1 file changed, 49 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/scsi/scsi_transport_iscsi.c b/drivers/scsi/scsi_transport_iscsi.c
> index 17a45716a0fe..d99c17306dff 100644
> --- a/drivers/scsi/scsi_transport_iscsi.c
> +++ b/drivers/scsi/scsi_transport_iscsi.c
> @@ -1616,6 +1616,12 @@ static DECLARE_TRANSPORT_CLASS(iscsi_connection_class,
> static struct sock *nls;
> static DEFINE_MUTEX(rx_queue_mutex);
>
> +/*
> + * conn_mutex protects the {start,bind,stop,destroy}_conn from racing
> + * against the kernel stop_connection recovery mechanism
> + */
> +static DEFINE_MUTEX(conn_mutex);
> +
> static LIST_HEAD(sesslist);
> static LIST_HEAD(sessdestroylist);
> static DEFINE_SPINLOCK(sesslock);
> @@ -2442,6 +2448,32 @@ int iscsi_offload_mesg(struct Scsi_Host *shost,
> }
> EXPORT_SYMBOL_GPL(iscsi_offload_mesg);
>
> +/*
> + * This can be called without the rx_queue_mutex, if invoked by the kernel
> + * stop work. But, in that case, it is guaranteed not to race with
> + * iscsi_destroy by conn_mutex.
> + */
> +static void iscsi_if_stop_conn(struct iscsi_cls_conn *conn, int flag)
> +{
> + /*
> + * It is important that this path doesn't rely on
> + * rx_queue_mutex, otherwise, a thread doing allocation on a
> + * start_session/start_connection could sleep waiting on a
> + * writeback to a failed iscsi device, that cannot be recovered
> + * because the lock is held. If we don't hold it here, the
> + * kernel stop_conn_work_fn has a chance to stop the broken
> + * session and resolve the allocation.
> + *
> + * Still, the user invoked .stop_conn() needs to be serialized
> + * with stop_conn_work_fn by a private mutex. Not pretty, but
> + * it works.
> + */
> + mutex_lock(&conn_mutex);
> + conn->transport->stop_conn(conn, flag);
> + mutex_unlock(&conn_mutex);
> +
> +}
> +
> static void stop_conn_work_fn(struct work_struct *work)
> {
> struct iscsi_cls_conn *conn, *tmp;
> @@ -2460,30 +2492,17 @@ static void stop_conn_work_fn(struct work_struct *work)
> uint32_t sid = iscsi_conn_get_sid(conn);
> struct iscsi_cls_session *session;
>
> - mutex_lock(&rx_queue_mutex);
> -
> session = iscsi_session_lookup(sid);
> if (session) {
> if (system_state != SYSTEM_RUNNING) {
> session->recovery_tmo = 0;
> - conn->transport->stop_conn(conn,
> - STOP_CONN_TERM);
> + iscsi_if_stop_conn(conn, STOP_CONN_TERM);
> } else {
> - conn->transport->stop_conn(conn,
> - STOP_CONN_RECOVER);
> + iscsi_if_stop_conn(conn, STOP_CONN_RECOVER);
> }
> }
>
> list_del_init(&conn->conn_list_err);
> -
> - mutex_unlock(&rx_queue_mutex);
> -
> - /* we don't want to hold rx_queue_mutex for too long,
> - * for instance if many conns failed at the same time,
> - * since this stall other iscsi maintenance operations.
> - * Give other users a chance to proceed.
> - */
> - cond_resched();
> }
> }
>
> @@ -2843,8 +2862,11 @@ iscsi_if_destroy_conn(struct iscsi_transport *transport, struct iscsi_uevent *ev
> spin_unlock_irqrestore(&connlock, flags);
>
> ISCSI_DBG_TRANS_CONN(conn, "Destroying transport conn\n");
> +
> + mutex_lock(&conn_mutex);
> if (transport->destroy_conn)
> transport->destroy_conn(conn);
> + mutex_unlock(&conn_mutex);
>
> return 0;
> }
> @@ -3686,9 +3708,12 @@ iscsi_if_recv_msg(struct sk_buff *skb, struct nlmsghdr *nlh, uint32_t *group)
> break;
> }
>
> + mutex_lock(&conn_mutex);
> ev->r.retcode = transport->bind_conn(session, conn,
> ev->u.b_conn.transport_eph,
> ev->u.b_conn.is_leading);
> + mutex_unlock(&conn_mutex);
> +
> if (ev->r.retcode || !transport->ep_connect)
> break;
>
> @@ -3709,25 +3734,31 @@ iscsi_if_recv_msg(struct sk_buff *skb, struct nlmsghdr *nlh, uint32_t *group)
> break;
> case ISCSI_UEVENT_START_CONN:
> conn = iscsi_conn_lookup(ev->u.start_conn.sid, ev->u.start_conn.cid);
> - if (conn)
> + if (conn) {
> + mutex_lock(&conn_mutex);
> ev->r.retcode = transport->start_conn(conn);
> + mutex_unlock(&conn_mutex);
> + }
> else
> err = -EINVAL;
> break;
> case ISCSI_UEVENT_STOP_CONN:
> conn = iscsi_conn_lookup(ev->u.stop_conn.sid, ev->u.stop_conn.cid);
> if (conn)
> - transport->stop_conn(conn, ev->u.stop_conn.flag);
> + iscsi_if_stop_conn(conn, ev->u.stop_conn.flag);
> else
> err = -EINVAL;
> break;
> case ISCSI_UEVENT_SEND_PDU:
> conn = iscsi_conn_lookup(ev->u.send_pdu.sid, ev->u.send_pdu.cid);
> - if (conn)
> + if (conn) {
> + mutex_lock(&conn_mutex);
> ev->r.retcode = transport->send_pdu(conn,
> (struct iscsi_hdr*)((char*)ev + sizeof(*ev)),
> (char*)ev + sizeof(*ev) + ev->u.send_pdu.hdr_size,
> ev->u.send_pdu.data_size);
> + mutex_unlock(&conn_mutex);
> + }
> else
> err = -EINVAL;
> break;
--
Martin K. Petersen Oracle Linux Engineering
