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authorDan Williams <dan.j.williams@intel.com>2008-04-07 18:35:01 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2008-04-19 22:10:29 -0400
commit2424b5dd062cbe3e0578ae7b11a1b360ad22f451 (patch)
tree75b6d67412969074237a371947280b419c5d43db /Documentation/filesystems/sysfs.txt
parentb844eba292b477cda14582bfc6f535deed57a82d (diff)
sysfs: refill attribute buffer when reading from offset 0
Requiring userspace to close and re-open sysfs attributes has been the policy since before 2.6.12. It allows userspace to get a consistent snapshot of kernel state and consume it with incremental reads and seeks. Now, if the file position is zero the kernel assumes userspace wants to see the new value. The application for this change is to allow a userspace RAID metadata handler to check the state of an array without causing any memory allocations. Thus not causing writeback to a raid array that might be blocked waiting for userspace to take action. Cc: Neil Brown <neilb@suse.de> Acked-by: Tejun Heo <htejun@gmail.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/filesystems/sysfs.txt')
-rw-r--r--Documentation/filesystems/sysfs.txt9
1 files changed, 7 insertions, 2 deletions
diff --git a/Documentation/filesystems/sysfs.txt b/Documentation/filesystems/sysfs.txt
index 4598ef7b622b..7f27b8f840d0 100644
--- a/Documentation/filesystems/sysfs.txt
+++ b/Documentation/filesystems/sysfs.txt
@@ -176,8 +176,10 @@ implementations:
176 Recall that an attribute should only be exporting one value, or an 176 Recall that an attribute should only be exporting one value, or an
177 array of similar values, so this shouldn't be that expensive. 177 array of similar values, so this shouldn't be that expensive.
178 178
179 This allows userspace to do partial reads and seeks arbitrarily over 179 This allows userspace to do partial reads and forward seeks
180 the entire file at will. 180 arbitrarily over the entire file at will. If userspace seeks back to
181 zero or does a pread(2) with an offset of '0' the show() method will
182 be called again, rearmed, to fill the buffer.
181 183
182- On write(2), sysfs expects the entire buffer to be passed during the 184- On write(2), sysfs expects the entire buffer to be passed during the
183 first write. Sysfs then passes the entire buffer to the store() 185 first write. Sysfs then passes the entire buffer to the store()
@@ -192,6 +194,9 @@ implementations:
192 194
193Other notes: 195Other notes:
194 196
197- Writing causes the show() method to be rearmed regardless of current
198 file position.
199
195- The buffer will always be PAGE_SIZE bytes in length. On i386, this 200- The buffer will always be PAGE_SIZE bytes in length. On i386, this
196 is 4096. 201 is 4096.
197 202