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    Subject[PATCH 10/12] relayfs: add Documentation on global relay buffers
    Documentation update for creating global buffers.

    Signed-off-by: Tom Zanussi <>


    relayfs.txt | 21 ++++++++++++++++++++-
    1 files changed, 20 insertions(+), 1 deletion(-)

    diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt
    --- a/Documentation/filesystems/relayfs.txt
    +++ b/Documentation/filesystems/relayfs.txt
    @@ -143,7 +143,7 @@ Here's a summary of the API relayfs prov
    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)

    helper functions:
    @@ -367,6 +367,25 @@ create_buf_file() is defined, remove_buf
    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.

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