Registering the same notifier to a hook repeatedly can cause the hook
list to form a ring or lose other members of the list.

  case1: An infinite loop in notifier_chain_register() can cause soft lockup
          atomic_notifier_chain_register(&test_notifier_list, &test1);
          atomic_notifier_chain_register(&test_notifier_list, &test1);
          atomic_notifier_chain_register(&test_notifier_list, &test2);

  case2: An infinite loop in notifier_chain_register() can cause soft lockup
          atomic_notifier_chain_register(&test_notifier_list, &test1);
          atomic_notifier_chain_register(&test_notifier_list, &test1);
          atomic_notifier_call_chain(&test_notifier_list, 0, NULL);

  case3: lose other hook test2
          atomic_notifier_chain_register(&test_notifier_list, &test1);
          atomic_notifier_chain_register(&test_notifier_list, &test2);
          atomic_notifier_chain_register(&test_notifier_list, &test1);

  case4: Unregister returns 0, but the hook is still in the linked list,
         and it is not really registered. If you call
         notifier_call_chain after ko is unloaded, it will trigger oops.

If the system is configured with softlockup_panic and the same hook is
repeatedly registered on the panic_notifier_list, it will cause a loop
panic.

Add a check in notifier_chain_register(), intercepting duplicate
registrations to avoid infinite loops

Link: http://lkml.kernel.org/r/1568861888-34045-2-git-send-email-nixiaoming@huawei.comSigned-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Cc: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>