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.Dd February 25, 2008
.Os
.Dt VOP_LOCK 9
.Sh NAME
.Nm VOP_LOCK ,
.Nm VOP_UNLOCK ,
.Nm VOP_ISLOCKED ,
.Nm vn_lock
.Nd serialize access to a vnode
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.h
.In sys/vnode.h
.Ft int
.Fn VOP_LOCK "struct vnode *vp" "int flags"
.Ft int
.Fn VOP_UNLOCK "struct vnode *vp" "int flags"
.Ft int
.Fn VOP_ISLOCKED "struct vnode *vp"
.Ft int
.Fn vn_lock "struct vnode *vp" "int flags"
.Sh DESCRIPTION
These calls are used to serialize access to the file system, such as
to prevent two writes to the same file from happening at the
same time.
.Pp
The arguments are:
.Bl -tag -width flags
.It Fa vp
The vnode being locked or unlocked.
.It Fa flags
One of the lock request types:
.Pp
.Bl -tag -width ".Dv LK_CANRECURSE" -offset indent -compact
.It Dv LK_SHARED
Shared lock.
.It Dv LK_EXCLUSIVE
Exclusive lock.
.It Dv LK_UPGRADE
Shared-to-exclusive upgrade.
.It Dv LK_DOWNGRADE
Exclusive-to-shared downgrade.
.It Dv LK_RELEASE
Release any type of lock.
.It Dv LK_DRAIN
Wait for all lock activity to end.
.El
.Pp
The lock type may be
.Em or Ns 'ed
with these lock flags:
.Pp
.Bl -tag -width ".Dv LK_CANRECURSE" -offset indent -compact
.It Dv LK_NOWAIT
Do not sleep to wait for lock.
.It Dv LK_SLEEPFAIL
Sleep, then return failure.
.It Dv LK_CANRECURSE
Allow recursive exclusive lock.
.It Dv LK_NOWITNESS
Instruct
.Xr witness 4
to ignore this instance.
.El
.Pp
The lock type may be
.Em or Ns 'ed
with these control flags:
.Pp
.Bl -tag -width ".Dv LK_CANRECURSE" -offset indent -compact
.It Dv LK_INTERLOCK
Specify when the caller already has a simple lock
.Fn ( VOP_LOCK
will unlock the simple lock after getting the lock).
.It Dv LK_RETRY
Retry until locked.
.El
.Pp
Kernel code should use
.Fn vn_lock
to lock a vnode rather than calling
.Fn VOP_LOCK
directly.
.Fn vn_lock
also does not want a thread specified as argument but it
assumes curthread to be used.
.Sh RETURN VALUES
Zero is returned on success, otherwise an error is returned.
.Sh PSEUDOCODE
.Bd -literal
struct vopnode {
    int von_flag;
    /*
     * Other file system specific data.
     */
    ...;
};
#define VON_LOCKED	1
#define VON_WANTED	2
#define VTOVON(vp)	((struct vopnode *) (vp)->v_data)

int
vop_lock(struct vnode *vp)
{
    struct vopnode* vop;

start:
    while (vp->v_flag & VXLOCK) {
	vp->v_flag |= VXWANT;
	tsleep((caddr_t)vp, PINOD, "voplk1", 0);
    }
    if (vp->v_tag == VT_NON)
	return ENOENT;

    vop = VTOVON(vp);
    if (vop->von_flag & VON_LOCKED) {
	vop->von_flag |= VON_WANTED;
	tsleep((caddr_t) vop, PINOD, "voplk2", 0);
	goto start;
    }

    vop->von_flag |= VON_LOCKED;

    return 0;
}

int
vop_unlock(struct vnode *vp)
{
    struct vopnode *vop = VTOVON(vp);

    if ((vop->von_flag & VON_LOCKED) == 0) {
	panic("vop_unlock not locked");
    }
    vop->von_flag &= ~VON_LOCKED;
    if (vop->von_flag & VON_WANTED) {
	vop->von_flag &= ~VON_WANTED;
	wakeup((caddr_t) vop);
    }

    return 0;
}

int
vop_islocked(struct vnode *vp)
{
    struct vopnode *vop = VTOVON(vp);

    if (vop->von_flag & VON_LOCKED)
	return 1;
    else
	return 0;
}
.Ed
.Sh SEE ALSO
.Xr vnode 9
.Sh AUTHORS
This manual page was written by
.An Doug Rabson .