VN_LOCK(9) BSD Kernel Manual VN_LOCK(9)
vn_lock - acquire the vnode lock
#include <sys/types.h> #include <sys/vnode.h> int 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- fined. MirOS BSD #10-current March 9, 2001 1
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