spinlock contention in ext4_getattr()

Issue

• spinlock contention in ext4_getattr()
• If ls is issued from multiple vsftpd daemon processes on a directory with a large number of files in it (like 80,000) they all contend on the i_block_reservation_lock spin lock as they all attempt to get details of the files in the directory. This effectively single threads all of the processes and produces a high contention rate on each inodes spinlock in series.

perf result

80.07%         vsftpd  [kernel.kallsyms]     [k] _spin_lock                 <==
 5.03%         vsftpd  ^E                     [.] 0x0000000000089745
 1.06%         vsftpd  [kernel.kallsyms]     [k] _atomic_dec_and_lock
 1.00%         :15904  [kernel.kallsyms]     [k] _spin_lock
 1.00%         :15918  [kernel.kallsyms]     [k] _spin_lock