Commit df49af8f authored by Tom Zanussi's avatar Tom Zanussi Committed by Linus Torvalds
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[PATCH] relayfs: add Documentation on global relay buffers

Documentation update for creating global buffers.
Signed-off-by: default avatarTom Zanussi <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent e6c08367
......@@ -143,7 +143,7 @@ Here's a summary of the API relayfs provides to in-kernel clients:
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_file() must also be defined;
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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