Date: Tue, 10 Sep 2002 04:54:39 -0700 (PDT) From: Don Lewis <dl-freebsd@catspoiler.org> To: bde@zeta.org.au Cc: rwatson@FreeBSD.ORG, current@FreeBSD.ORG, jeff@FreeBSD.ORG Subject: Re: vnode lock assertion problem in nfs_link() Message-ID: <200209101154.g8ABsdwr094898@gw.catspoiler.org> In-Reply-To: <200209101046.g8AAk7wr094789@gw.catspoiler.org>
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On 10 Sep, Don Lewis wrote:
> On 10 Sep, Bruce Evans wrote:
>> On Tue, 10 Sep 2002, Don Lewis wrote:
>>
>>> On 10 Sep, Bruce Evans wrote:
>
>>> > The changes are obviously just cleanups for leaf file systems, but I
>>> > wonder why everything wasn't always locked at the top. Could it have
>>> > been because locking all the way down is harmful?
>> I think this is because some places are still worrying about using
>> link(2) on directories. I think there aren't any significant problems
>> when vp is not a directory. Hard linking of directories was permitted
>> (and perhaps even supported) until relatively recently:
>
> I suspect that you are right about this. If so, it should be able to
> let namei() lock the first vnode since we check its type and bail out if
> it is a directory before the second namei() call.
I looked at namei() and ufs_lookup() and found a problem with telling
the first namei() to grab the lock. It appears that the *_lookup()
routines always lock the child vnode, and namei() later releases the
lock if LOCKLEAF is not set. If we passed LOCKLEAF to the first
namei(), then link("/foo/bar", "/foo/bar") would attempt to lock "bar"
twice.
Because we now bail out early if the first vnode turns out to be a
pointer, I believe we can dispense with the vnode inequality test and
always grab the lock.
BTW, is it safe to call ASSERT_VOP_UNLOCKED() in the SMP case after the
reference has been dropped with vput() or vrele()?
Here's a replacement for the kern_link() patch in my previous patch
file:
Index: vfs_syscalls.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.285
diff -u -r1.285 vfs_syscalls.c
--- vfs_syscalls.c 1 Sep 2002 20:37:28 -0000 1.285
+++ vfs_syscalls.c 10 Sep 2002 11:28:01 -0000
@@ -1027,10 +1027,12 @@
if (nd.ni_vp != NULL) {
vrele(nd.ni_vp);
error = EEXIST;
- } else {
+ } else if ((error = vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td))
+ == 0) {
VOP_LEASE(nd.ni_dvp, td, td->td_ucred, LEASE_WRITE);
VOP_LEASE(vp, td, td->td_ucred, LEASE_WRITE);
error = VOP_LINK(nd.ni_dvp, vp, &nd.ni_cnd);
+ VOP_UNLOCK(vp, 0, td);
}
NDFREE(&nd, NDF_ONLY_PNBUF);
vput(nd.ni_dvp);
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