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NAME

     VOP_LOCK, VOP_UNLOCK, VOP_ISLOCKED, vn_lock - serialize access to a vnode

SYNOPSIS

     #include <sys/param.h>
     #include <sys/lock.h>
     #include <sys/vnode.h>

     int
     VOP_LOCK(struct vnode *vp, int flags, struct thread *td);

     int
     VOP_UNLOCK(struct vnode *vp, int flags, struct thread *td);

     int
     VOP_ISLOCKED(struct vnode *vp, struct thread *td);

     int
     vn_lock(struct vnode *vp, int flags, struct thread *td);

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.

     The arguments are:

     vp     The vnode being locked or unlocked.

     flags  One of the lock request types:

                  LK_SHARED       Shared lock.
                  LK_EXCLUSIVE    Exclusive lock.
                  LK_UPGRADE      Shared-to-exclusive upgrade.
                  LK_EXCLUPGRADE  First shared-to-exclusive upgrade.
                  LK_DOWNGRADE    Exclusive-to-shared downgrade.
                  LK_RELEASE      Release any type of lock.
                  LK_DRAIN        Wait for all lock activity to end.

            The lock type may be or’ed with these lock flags:

                  LK_NOWAIT       Do not sleep to wait for lock.
                  LK_SLEEPFAIL    Sleep, then return failure.
                  LK_CANRECURSE   Allow recursive exclusive lock.
                  LK_REENABLE     Lock is to be reenabled after drain.
                  LK_NOPAUSE      No spinloop.

            The lock type may be or’ed with these control flags:

                  LK_INTERLOCK    Specify when the caller already has a simple
                                  lock (VOP_LOCK() will unlock the simple lock
                                  after getting the lock).
                  LK_RETRY        Retry until locked.
                  LK_NOOBJ        Do not create object.

     td     Thread context to use for the locks.

     Kernel code should use vn_lock() to lock a vnode rather than calling
     VOP_LOCK() directly.

RETURN VALUES

     Zero is returned on success, otherwise an error is returned.

PSEUDOCODE

     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;
     }

SEE ALSO

     vnode(9)

AUTHORS

     This manual page was written by Doug Rabson.