NAME

fstrans, fstrans_setstate, fstrans_getstate, fstrans_start, fstrans_start_nowait, fstrans_done, fstrans_is_owner, fscow_establish, fscow_disestablish, fscow_run — file system suspension helper subsystem

SYNOPSIS

#include <sys/mount.h>
#include <sys/fstrans.h>

int
fstrans_setstate(struct mount *mp, enum fstrans_state new_state);

enum fstrans_state
fstrans_getstate(struct mount *mp);

void
fstrans_start(struct mount *mp, enum fstrans_lock_type lock_type);

int
fstrans_start_nowait(struct mount *mp, enum fstrans_lock_type lock_type);

void
fstrans_done(struct mount *mp);

int
fstrans_is_owner(struct mount *mp);

int
fscow_establish(struct mount *mp, int (*func)(void *, struct buf *, bool), void *cookie);

int
fscow_disestablish(struct mount *mp, int (*func)(void *, struct buf *, bool), void *cookie);

int
fscow_run(struct buf *bp, bool data_valid);

DESCRIPTION

The fstrans subsystem is a set of operations to assist file system suspension. These operations must not be used outside of file systems.

File systems supporting this subsystem must set the flag IMNT_HAS_TRANS in mnt_iflag.

File systems are always in one of these states:

FSTRANS_NORMAL

Normal operations.

FSTRANS_SUSPENDING

Preparing a suspension.

FSTRANS_SUSPENDED

Suspended.

This state is represented by enum fstrans_state.

All file system operations use a fstrans lock. This lock is recursive. A thread already owning a lock will always get another lock. The lock has two variants:

FSTRANS_SHARED

A lock that will be granted if the file system is in state FSTRANS_NORMAL.

FSTRANS_LAZY

A lock that will be granted if the file system is in state FSTRANS_NORMAL or FSTRANS_SUSPENDING. It needs special care because operations using this variant will not block while the file system prepares suspension.

The lock variant is represented by enum fstrans_lock_type.

FUNCTIONS

The following functions comprise the API provided by fstrans.

fstrans_getstate(mp)

Returns the current state of the file system mp.

fstrans_setstate(mp, new_state)

Changes the state of the file system mp to new_state.

fstrans_start(mp, lock_type)

Sets a lock of type lock_type on the file system mp.

fstrans_start_nowait(mp, lock_type)

Like fstrans_start(), but will not wait for a state change of the file system when attempting to acquire the lock. The thread may still sleep while attempting to acquire the lock.

fstrans_done(mp)

Releases a lock on the file system mp.

fstrans_is_owner(mp)

Returns true if this thread is currently suspending the file system mp.

fscow_establish(mp, func, cookie)

Establish a copy-on-write callback for the file system mp. The function func will be called for every buffer written through this file system.

fscow_disestablish(mp, func, cookie)

Disestablish a copy-on-write callback registered with fscow_establish().

fscow_run(bp, data_valid)

Run all copy-on-write callbacks established for the file system this buffer belongs to. If data_valid is true the buffer data has not yet been modified.

RETURN VALUES

The functions fstrans_setstate() and fstrans_start_nowait() return zero on success and an error value on failure.

EXAMPLES

The following is an example of a file system suspend operation.

int 
xxx_suspendctl(struct mount *mp, int cmd) 
{ 
	int error; 
 
	switch (cmd) { 
	case SUSPEND_SUSPEND: 
		error = fstrans_setstate(mp, FSTRANS_SUSPENDING); 
		if (error != 0) 
			return error; 
 
		/* Sync file system state to disk. */ 
 
		return fstrans_setstate(mp, FSTRANS_SUSPENDED); 
 
	case SUSPEND_RESUME: 
		return fstrans_setstate(mp, FSTRANS_NORMAL); 
 
	default: 
		return EINVAL; 
	} 
}

This is an example of a file system operation.

int 
xxx_create(void *v) 
{ 
	struct vop_create_args *ap = v; 
	struct mount *mp = ap->a_dvp->v_mount; 
	int error; 
 
	if ((error = fstrans_start(mp, FSTRANS_SHARED)) != 0) 
		return error; 
 
	/* Actually create the node. */ 
 
	fstrans_done(mp); 
 
	return 0; 
}

CODE REFERENCES

The actual code implementing this subsystem can be found in the file sys/kern/vfs_trans.c.

SEE ALSO

vfs_resume(9), vfs_suspend(9)

HISTORY

The fstrans subsystem appeared in NetBSD 5.0.

AUTHORS

The fstrans subsystem was written by Jürgen Hannken-Illjes <hannken@NetBSD.org>.