IB/core: Don't register each MAD agent for LSM notifier

When creating many MAD agents in a short period of time, receive packet
processing can be delayed long enough to cause timeouts while new agents
are being added to the atomic notifier chain with IRQs disabled.  Notifier
chain registration and unregstration is an O(n) operation. With large
numbers of MAD agents being created and destroyed simultaneously the CPUs
spend too much time with interrupts disabled.

Instead of each MAD agent registering for it's own LSM notification,
maintain a list of agents internally and register once, this registration
already existed for handling the PKeys. This list is write mostly, so a
normal spin lock is used vs a read/write lock. All MAD agents must be
checked, so a single list is used instead of breaking them down per
device.

Notifier calls are done under rcu_read_lock, so there isn't a risk of
similar packet timeouts while checking the MAD agents security settings
when notified.

Signed-off-by: Daniel Jurgens <[email protected]>
Reviewed-by: Parav Pandit <[email protected]>
Signed-off-by: Leon Romanovsky <[email protected]>
Acked-by: Paul Moore <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

include/rdma/ib_mad.h

@@ -617,11 +617,10 @@ struct ib_mad_agent {
 	u32			hi_tid;
 	u32			flags;
 	void			*security;
-	struct notifier_block   lsm_nb;
+	struct list_head	mad_agent_sec_list;
 	u8			port_num;
 	u8			rmpp_version;
 	bool			smp_allowed;
-	bool			lsm_nb_reg;
 };
 
 /**