relay interface bugfixes and early logging support

This patch series was created while working on kmemtrace, a project that needs to do early tracing in the kernel, but the bugfixes are relevant to any project that uses the relay interface.

Only the third patch introduces a new feature, that is, buffers that have no associated files. This is exactly what kmemtrace needs, although other tracing projects can use this new API to trace early code and export the data as soon as VFS is ready.

Mathieu Desnoyers from LTTng has already expressed his interest in this functionality.

This, along with kmemtrace, is part of my work within this year's GSoC, mentored by Pekka Enberg.

Eduard - Gabriel Munteanu (3):
relay: Fix 4 off-by-one errors occuring when writing to a CPU buffer.
relay: Fix race condition which occurs when reading across CPUs.
relay: Add buffer-only channels; useful for early logging.

 Documentation/filesystems/relay.txt |   11 +++
 include/linux/relay.h               |   19 ++++--
 kernel/relay.c                      |  133 ++++++++++++++++++++++++++---------
 3 files changed, 125 insertions(+), 38 deletions(-)