commit e30f53aa upstream.

On a !PREEMPT kernel, attempting to use trace-cmd results in a soft
lockup:

 # trace-cmd record -e raw_syscalls:* -F false
 NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [trace-cmd:61]
 ...
 Call Trace:
  [<ffffffff8105b580>] ? __wake_up_common+0x90/0x90
  [<ffffffff81092e25>] wait_on_pipe+0x35/0x40
  [<ffffffff810936e3>] tracing_buffers_splice_read+0x2e3/0x3c0
  [<ffffffff81093300>] ? tracing_stats_read+0x2a0/0x2a0
  [<ffffffff812d10ab>] ? _raw_spin_unlock+0x2b/0x40
  [<ffffffff810dc87b>] ? do_read_fault+0x21b/0x290
  [<ffffffff810de56a>] ? handle_mm_fault+0x2ba/0xbd0
  [<ffffffff81095c80>] ? trace_event_buffer_lock_reserve+0x40/0x80
  [<ffffffff810951e2>] ? trace_buffer_lock_reserve+0x22/0x60
  [<ffffffff81095c80>] ? trace_event_buffer_lock_reserve+0x40/0x80
  [<ffffffff8112415d>] do_splice_to+0x6d/0x90
  [<ffffffff81126971>] SyS_splice+0x7c1/0x800
  [<ffffffff812d1edd>] tracesys_phase2+0xd3/0xd8

The problem is this: tracing_buffers_splice_read() calls
ring_buffer_wait() to wait for data in the ring buffers.  The buffers
are not empty so ring_buffer_wait() returns immediately.  But
tracing_buffers_splice_read() calls ring_buffer_read_page() with full=1,
meaning it only wants to read a full page.  When the full page is not
available, tracing_buffers_splice_read() tries to wait again with
ring_buffer_wait(), which again returns immediately, and so on.

Fix this by adding a "full" argument to ring_buffer_wait() which will
make ring_buffer_wait() wait until the writer has left the reader's
page, i.e.  until full-page reads will succeed.

Link: http://lkml.kernel.org/r/1415645194-25379-1-git-send-email-rabin@rab.in

Fixes: b1169cc6 ("tracing: Remove mock up poll wait function")
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>