spinlock contention of files->file_lock

[Eric Dumazet]

Speaking of spinlock contention, the files->file_lock is a good example.

Multi threaded programs hit this spinlock three times per fd : 

alloc_fd() / fd_install() / close() 

I think fd_install() could be done without this spinlock.

we want to protect fd_install() from a concurrent resize of the fd
table.

We could either :

- Add a spinlock used for resize, and fd_install().
  A bit suboptimal but easy to code and review.

- Add a seqcount, and cmpxchg() to synchronize fd_install() with the
potential resizer. (Might need additional RCU locking)



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