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 interlock).
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 interlock
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.
vnode(9)
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
discipline may actually save lines of code, esp. if the number of file
systems 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 defined.
OpenBSD 3.6 March 9, 2001
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