Date: Wed, 21 Jan 1998 18:56:27 +1030 From: Greg Lehey <grog@lemis.com> To: FreeBSD Hackers <hackers@FreeBSD.ORG> Subject: Locking on disk slice I/O--yes, no or how? Message-ID: <19980121185627.21744@lemis.com>
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I'm currently trying to perform low-level I/O to disk slices in a
driver. I've read section 9 of the manual, which tells me that all
reads and writes should be protected with a VOP_LOCK/VOP_UNLOCK pair.
I've tried this, and get a panic: "lockmgr: locking against myself"
Looking at the dump, I find that my vnode does not appear to be
locked. Here it is without most of the fields.
(kgdb) p *devvp
$12 = {
v_flag = 0,
v_usecount = 1,
v_writecount = 0,
v_type = VCHR,
v_un = {
vu_mountedhere = 0xf0337820,
vu_socket = 0xf0337820,
vu_specinfo = 0xf0337820,
vu_fifoinfo = 0xf0337820
},
v_interlock = {
lock_data = 0
},
v_vnlock = 0x0,
v_tag = VT_UFS,
v_data = 0xf0548700, /* this seems to be used by the locks */
}
}
The following code leads to a panic:
VOP_LOCK (devvp, LK_EXCLUSIVE, curproc); /* lock the vnode */
error = VOP_WRITE (devvp, &uio, IO_NODELOCKED, FSCRED); /* write the header */
VOP_UNLOCK (devvp, 0, curproc); /* and unlock it again */
#1 0xf011bdaf in panic (fmt=0xf01172cf "lockmgr: locking against myself") at ../../kern/kern_shutdown.c:425
#2 0xf0117540 in lockmgr (lkp=0xf0548700, flags=2, interlkp=0xf052365c, p=0xf04fda00) at ../../kern/kern_lock.c:288
#3 0xf0138071 in vop_stdlock (ap=0xf3a45cac) at ../../kern/vfs_default.c:194
#4 0xf01c1a41 in ufs_vnoperatespec (ap=0xf3a45cac) at ../../ufs/ufs/ufs_vnops.c:2186
#5 0xf3cec47e in save_config () at vnode_if.h:811
Looking at the stack frame for vop_stdlock, it looks as if I'm trying
to use my devvp->v_data as lock data to lock against my own vnode. I
can't make sense of this (except that, under these circumstances, I
can understand the panic). I have a number of questions:
1. What is the lock manager trying to do here? What's the content of
devvp->v_data?
2. Why do I need to lock at all? According to the 4.4BSD book, it's
"advisory", and then only for directory searches and such.
3. Are the parameters correct? It looks as if I shouldn't specify
curproc in the calls, but I can't find any documentation that
tells me what to put there, nor indeed what use the whole thing
is.
4. This device is /dev/rsd1e. Should I even be calling
ufs_vnoperatespec?
5. Am I doing anything else obviously wrong?
Greg
help
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