FSTRANS(9)

FSTRANS(9) | Kernel Developer's Manual | FSTRANS(9)

# 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>.

April 13, 2010 | NetBSD 7.1