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[PATCH] relayfs: add Documentation on global relay buffers
Documentation update for creating global buffers. Signed-off-by: Tom Zanussi <zanussi@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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@ -143,7 +143,7 @@ Here's a summary of the API relayfs provides to in-kernel clients:
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subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
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subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
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buf_mapped(buf, filp)
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buf_mapped(buf, filp)
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buf_unmapped(buf, filp)
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buf_unmapped(buf, filp)
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create_buf_file(filename, parent, mode, buf)
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create_buf_file(filename, parent, mode, buf, is_global)
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remove_buf_file(dentry)
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remove_buf_file(dentry)
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helper functions:
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helper functions:
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@ -367,6 +367,25 @@ create_buf_file() is defined, remove_buf_file() must also be defined;
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it's responsible for deleting the file(s) created in create_buf_file()
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it's responsible for deleting the file(s) created in create_buf_file()
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and is called during relay_close().
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and is called during relay_close().
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The create_buf_file() implementation can also be defined in such a way
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as to allow the creation of a single 'global' buffer instead of the
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default per-cpu set. This can be useful for applications interested
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mainly in seeing the relative ordering of system-wide events without
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the need to bother with saving explicit timestamps for the purpose of
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merging/sorting per-cpu files in a postprocessing step.
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To have relay_open() create a global buffer, the create_buf_file()
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implementation should set the value of the is_global outparam to a
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non-zero value in addition to creating the file that will be used to
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represent the single buffer. In the case of a global buffer,
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create_buf_file() and remove_buf_file() will be called only once. The
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normal channel-writing functions e.g. relay_write() can still be used
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- writes from any cpu will transparently end up in the global buffer -
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but since it is a global buffer, callers should make sure they use the
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proper locking for such a buffer, either by wrapping writes in a
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spinlock, or by copying a write function from relayfs_fs.h and
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creating a local version that internally does the proper locking.
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See the 'exported-relayfile' examples in the relay-apps tarball for
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See the 'exported-relayfile' examples in the relay-apps tarball for
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examples of creating and using relay files in debugfs.
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examples of creating and using relay files in debugfs.
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