On Sun, 3 Jun 2012, Konstantin Belousov wrote:
On Sat, Jun 02, 2012 at 11:34:21PM +0000, Benjamin Kaduk wrote:
Hi all,
This caused me to take a look at the namecache functions and find not very
much documentation of their use. I see that NetBSD has had a namecache.9
man page since 2001 [2], and their actual code shares its origin from 4.2
BSD with ours ... but our vfs_cache.c is at (CVS) revision 1.181 versus
NetBSD's 1.88, and there are at least superficial differences between the
two.
Superficial ? How can you judge ?
Well, the first thing that came to mind was that we have struct
namecache_ts that was recently added. It looks like NetBSD also has
per-cpu namecache data (I just noticed). They also use explicit mutex
operations on the namecache_lock as opposed to our macros around the
rwlock cache_lock. I have not put in the time to tell whether these
represent significant structural differences or just superficial renaming.
Is there a summary of the design/implementation somewhere that I could use
That sounds like 'no'.
as a reference, or can someone send one here? I would like to come up
with a namecache.9 for FreeBSD's implementation.
The particular question which promped my investigation is for the locking
around cache_purge(); I was not immediately able to tell from looking at
other consumers whether the vnode lock needs to or should be held around
such calls. Looking at the implementation of some of the other namecache
functions makes it seem that the lock order is vnode lock then namecache
lock, which would suggest that holding the vnode lock across cache_purge
would be okay, but I would like some confirmation before I use and
document it as such.
Vnode lock is not needed around cache_purge(). This is in fact documented
by not making asserts for lock ownership inside the function.
I was not sufficiently confident to rely on such negative documentation
from code.
The cache_purge() does not operate on any fs-specific data associated
with the vnode, and the data it does operate on is protected by
namecache rwlock. The only guarantee which cache_purge() needs from its
caller is that passed vnode pointer remains valid for duration of the
call, which is normally ensured by holding the vnode, often implicitely.
It seemed so to me; thank you for confirming.
The order between namecache rwlock and vnode lock is obvious, since vnode
lock is lockmgr/sleepable. On the other hand, vnode interlock is after
namecache rwlock.
I missed this, it is helpful to know.
Thanks,
Ben
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