MirOS Manual: vn_lock(9)

VN_LOCK(9)                    BSD Kernel Manual                     VN_LOCK(9)


     vn_lock - acquire the vnode lock


     #include <sys/types.h>
     #include <sys/vnode.h>

     vn_lock(struct vnode *vp, int flags, struct proc *p);


     The vn_lock() function is used to acquire the vnode lock. Certain file
     system operations require that the vnode lock be held when they are
     called. See sys/kern/vnode_if.src for more details.

     The vn_lock() function must not be called when the vnode's reference
     count is zero. Instead, the vget() function should be used.

     The flags argument may contain the following flags:

           LK_RETRY        Return the vnode even if it has been reclaimed.
           LK_INTERLOCK    Must be set if the caller owns the vnode interlock.
           LK_NOWAIT       Don't wait if the vnode lock is held by someone
                           else (may still wait on reclamation lock on or in-
                           terlock). Must not be used with LK_RETRY.
           LK_EXCLUSIVE    Acquire an exclusive lock.
           LK_SHARED       Acquire a shared lock.

     The vn_lock() function can sleep. The vn_lock() releases the vnode inter-
     lock before exit.


     Upon successful completion, a value of 0 is returned. Otherwise, one of
     the following errors is returned.


     [ENOENT]      The vnode has been reclaimed and is dead. This error is
                   only returned if the LK_RETRY flag is not passed.

     [EBUSY]       The LK_NOWAIT flag was set and vn_lock() would have slept.




     The locking discipline is bizarre. Many vnode operations are passed
     locked vnodes on entry but release the lock before they exit. Discussions
     with Kirk McKusick indicate that locking discipline evolved out of the
     pre-VFS way of doing inode locking. In addition, the current locking dis-
     cipline may actually save lines of code, esp. if the number of file sys-
     tems is fewer than the number of call sites. However, the VFS interface
     would require less wizardry if the locking discipline were simpler.

     The locking discipline is used in some places to attempt to make a series
     of operations atomic (e.g., permissions check + operation). This does not
     work for non-local file systems that do not support locking (e.g., NFS).

     Are vnode locks even necessary? The security checks can be moved into the
     individual file systems. Each file system can have the responsibility of
     ensuring that vnode operations are suitably atomic.

     The LK_NOWAIT flag does prevent the caller from sleeping.

     The locking discipline as it relates to shared locks has yet to be de-

MirOS BSD #10-current           March 9, 2001                                1

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