After some research I still hold that fsync blocks, at least on
FreeBSD. Am I missing something?
Here's the evidence:
Code from: /usr/src/sys/syscalls/vfs_syscalls
int
fsync(p, uap) struct proc *p; struct fsync_args /* { syscallarg(int) fd; } */ *uap;
{ register struct vnode *vp; struct file *fp; vm_object_t obj; int error;
if ((error = getvnode(p->p_fd, SCARG(uap, fd), &fp)) != 0) return (error); vp = (struct
vnode*)fp->f_data; vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p); if (VOP_GETVOBJECT(vp, &obj) == 0)
vm_object_page_clean(obj, 0, 0, 0); if ((error = VOP_FSYNC(vp, fp->f_cred, MNT_WAIT, p)) == 0 &&
vp->v_mount&& (vp->v_mount->mnt_flag & MNT_SOFTDEP) && bioops.io_fsync) error =
(*bioops.io_fsync)(vp); VOP_UNLOCK(vp, 0, p); return (error);
}
Notice the calls to:
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);..VOP_UNLOCK(vp, 0, p);
surrounding the call to VOP_FSYNC.
From the man pages for VOP_UNLOCK:
HEADER STUFF .....
VOP_LOCK(struct vnode *vp, int flags, struct proc *p);
int VOP_UNLOCK(struct vnode *vp, int flags, struct proc *p);
int VOP_ISLOCKED(struct vnode *vp, struct proc *p);
int vn_lock(struct vnode *vp, int flags, struct proc *p);
DESCRIPTION These calls are used to serialize access to the filesystem, such as to prevent two writes to the same
filefrom 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 Don't create object
p process context to use for the locks
Kernel code should use vn_lock() to lock a vnode rather than calling VOP_LOCK() directly.
