freebsd/share/man/man9/VOP_LOCK.9

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.Dd July 24, 1996
.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
.Fd #include <sys/param.h>
.Fd #include <sys/lock.h>
.Fd #include <sys/vnode.h>
.Ft int
.Fn VOP_LOCK "struct vnode *vp" "int flags" "struct proc *p"
.Ft int
.Fn VOP_UNLOCK "struct vnode *vp" "int flags" "struct proc *p"
.Ft int
.Fn VOP_ISLOCKED "struct vnode *vp" "struct proc *p"
.Ft int
.Fn vn_lock "struct vnode *vp" "int flags" "struct proc *p"
.Sh DESCRIPTION
.Pp
These calls are used to serialize access to the filesystem, 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 Ar vp
the vnode being locked or unlocked
.It Ar flags
One of the lock request types:
.Bl -column LK_EXCLUPGRADE -offset indent
.It Dv LK_SHARED Ta "Shared lock"
.It Dv LK_EXCLUSIVE Ta "Exclusive lock"
.It Dv LK_UPGRADE Ta "Shared-to-exclusive upgrade"
.It Dv LK_EXCLUPGRADE Ta "First shared-to-exclusive upgrade"
.It Dv LK_DOWNGRADE Ta "Exclusive-to-shared downgrade"
.It Dv LK_RELEASE Ta "Release any type of lock"
.It Dv LK_DRAIN Ta "Wait for all lock activity to end"
.El
.Pp
The lock type may be
.Em or Ns 'ed
with these lock flags:
.Bl -column LK_CANRECURSE -offset indent
.It Dv LK_NOWAIT Ta "Do not sleep to wait for lock"
.It Dv LK_SLEEPFAIL Ta "Sleep, then return failure"
.It Dv LK_CANRECURSE Ta "Allow recursive exclusive lock"
.It Dv LK_REENABLE Ta "Lock is to be reenabled after drain"
.It Dv LK_NOPAUSE Ta "No spinloop"
.El
.Pp
The lock type may be
.Em or Ns 'ed
with these control flags:
.Bl -column LK_INTERLOCK -offset indent
.It Dv LK_INTERLOCK Ta "Specify when the caller already has a simple lock \
(VOP_LOCK will unlock the simple lock after getting the lock)"
.It Dv LK_RETRY Ta "Retry until locked"
.It Dv LK_NOOBJ Ta "Don't create object"
.El
.It Ar p
process context to use for the locks
.El
.Pp
Kernel code should use
.Fn vn_lock
to lock a vnode rather than calling
.Fn VOP_LOCK
directly.
.Sh RETURN VALUES
Zero is returned on success, otherwise an error is returned.
.Sh PSEUDOCODE
.Bd -literal
struct vopnode {
    int von_flag;
    /*
     * Other filesystem 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 man page was written by
.An Doug Rabson .