Hi!

I tried to figure out the origin of the bug reported by Pekka Paalanen
about the broken pipe:
http://kerneltrap.org/mailarchive/linux-kernel/2008/9/15/3305224

When I add a trace_mark with the boot tracer, I had this same problem
but this time it was easy to reproduce.

When it calls a tracer's print_line callback, the print_trace_line function 
in trace.c returns whithout verifying if it could handle the entry properly.
And actually the seq could be empty.

For example the boot_tracer don't handle TRACE_PRINT. Nevertheless it
wants them to be printed as a default way.

So print_trace_line function should relay on the other functions which could handle
an output if one of them fail.

Reported-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
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