[PATCH] relayfs: add Documentation on global relay buffers

    subbuf_start(buf, subbuf, prev_subbuf, prev_padding)

    buf_mapped(buf, filp)

    buf_unmapped(buf, filp)

    create_buf_file(filename, parent, mode, buf)

    create_buf_file(filename, parent, mode, buf, is_global)

    remove_buf_file(dentry)

  helper functions:

it's responsible for deleting the file(s) created in create_buf_file()

and is called during relay_close().

The create_buf_file() implementation can also be defined in such a way

as to allow the creation of a single 'global' buffer instead of the

default per-cpu set.  This can be useful for applications interested

mainly in seeing the relative ordering of system-wide events without

the need to bother with saving explicit timestamps for the purpose of

merging/sorting per-cpu files in a postprocessing step.

To have relay_open() create a global buffer, the create_buf_file()

implementation should set the value of the is_global outparam to a

non-zero value in addition to creating the file that will be used to

represent the single buffer.  In the case of a global buffer,

create_buf_file() and remove_buf_file() will be called only once.  The

normal channel-writing functions e.g. relay_write() can still be used

- writes from any cpu will transparently end up in the global buffer -

but since it is a global buffer, callers should make sure they use the

proper locking for such a buffer, either by wrapping writes in a

spinlock, or by copying a write function from relayfs_fs.h and

creating a local version that internally does the proper locking.

See the 'exported-relayfile' examples in the relay-apps tarball for

examples of creating and using relay files in debugfs.