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[JFFS2][XATTR] Add a description about c->xattr_sem

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Add a description about the c->xattr_sem read/write semaphore
into README.Locking.

[3/10] jffs2-xattr-v5.1-03-append_README.Locking.patch

Signed-off-by: KaiGai Kohei <kaigai@ak.jp.nec.com>
This commit is contained in:
KaiGai Kohei 2006-05-13 15:14:14 +09:00
parent de1f72fab3
commit 8b0b339d46
1 changed files with 21 additions and 0 deletions
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21
fs/jffs2/README.Locking
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@ -150,3 +150,24 @@ the buffer.
Ordering constraints:
Lock wbuf_sem last, after the alloc_sem or and f->sem.
c->xattr_sem
------------
This read/write semaphore protects against concurrent access to the
xattr related objects which include stuff in superblock and ic->xref.
In read-only path, write-semaphore is too much exclusion. It's enough
by read-semaphore. But you must hold write-semaphore when updating,
creating or deleting any xattr related object.
Once xattr_sem released, there would be no assurance for the existence
of those objects. Thus, a series of processes is often required to retry,
when updating such a object is necessary under holding read semaphore.
For example, do_jffs2_getxattr() holds read-semaphore to scan xref and
xdatum at first. But it retries this process with holding write-semaphore
after release read-semaphore, if it's necessary to load name/value pair
from medium.
Ordering constraints:
Lock xattr_sem last, after the alloc_sem.