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[Eric Desrochers <eric.desrochers@xxxxxxxxxxxxx>]

This is a reproducer for the stall problem in drivers/tty/n_tty.c

To reproduce the problem, save the program below as pty.c, compile it,
and run it in parallel.

# cc -o pty pty.c
# for i in {1..16}; do ./pty& done; wait
The problem can be reproduced on a multi-socket server with recent CPUs.
The program always stalled during the first run when I used a server
with the following CPU.
  Intel(R) Xeon(R) CPU E5-2698 v3 @ 2.30GHz
  2-sockets x 16-cores x 2-threads

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https://bugs.launchpad.net/bugs/1512815

Title:
  14.04: read() from pty doesn't finish.

Status in linux package in Ubuntu:
  New

Bug description:
  It has been brought to my attention

  By the attached test program pty, a pair of process repeats writing
  and reading only '\n' to a master pseudoterminal device (/dev/ptmx)
  and a slave pseudoterminal device (/dev/pts/N) each. When we carry out
  the following 30 pairs 10,000 times, in a pair each process doesn't
  finish reading.

  $ pty 30
  The following message will be usually indicated immediately.
  #name copy num/sec usec/num
  pty_switch 30 1541842 0.648575

  When a message wasn't indicated any more, we got the attached dump file. 
  A kernel was 3.13.0-45.74. 
  A system was Ubuntu 14.04 LTS.

  The same problem occurred in case of 3.13.0-55.92 kernel and the following kernel have been tested as well:
  2.6.32, 3.10: not reproduces.
  3.19, 4.0.0, 4.1.3: reproduces.

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