332 lines
8.4 KiB
Groff
332 lines
8.4 KiB
Groff
.\" $NetBSD: fstrans.9,v 1.29 2018/10/05 16:21:22 uwe Exp $
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.\"
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.\" Copyright (c) 2007 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Juergen Hannken-Illjes.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd October 4, 2018
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.Dt FSTRANS 9
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.Os
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.Sh NAME
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.Nm fstrans ,
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.Nm fstrans_setstate ,
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.Nm fstrans_getstate ,
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.Nm fstrans_start ,
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.Nm fstrans_start_nowait ,
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.Nm fstrans_start_lazy ,
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.Nm fstrans_done ,
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.Nm fstrans_is_owner ,
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.Nm fscow_establish ,
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.Nm fscow_disestablish ,
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.Nm fscow_run
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.Nd file system suspension helper subsystem
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.Sh SYNOPSIS
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.In sys/mount.h
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.In sys/fstrans.h
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.Ft void
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.Fn fstrans_start "struct mount *mp"
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.Ft int
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.Fn fstrans_start_nowait "struct mount *mp"
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.Ft void
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.Fn fstrans_start_lazy "struct mount *mp"
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.Ft void
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.Fn fstrans_done "struct mount *mp"
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.Ft int
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.Fn fstrans_setstate "struct mount *mp" "enum fstrans_state new_state"
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.Ft "enum fstrans_state"
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.Fn fstrans_getstate "struct mount *mp"
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.Ft int
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.Fn fstrans_is_owner "struct mount *mp"
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.Ft int
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.Fn fscow_establish "struct mount *mp" \
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"int (*func)(void *, struct buf *, bool)" "void *cookie"
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.Ft int
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.Fn fscow_disestablish "struct mount *mp" \
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"int (*func)(void *, struct buf *, bool)" "void *cookie"
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.Ft int
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.Fn fscow_run "struct buf *bp" "bool data_valid"
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.Sh DESCRIPTION
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The
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.Nm
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subsystem assists file system suspension and copy-on-write snapshots.
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.Pp
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The file system's normal operations, such as its
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.Xr vnodeops 9 ,
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must be bracketed by
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.Fn fstrans_start
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and
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.Fn fstrans_done
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in a
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.Em shared
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transaction, which is blocked by suspending the file system and while
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it is suspended.
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.Pp
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Operations needed while suspending the file system must be bracketed by
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.Fn fstrans_start_lazy
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and
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.Fn fstrans_done
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in a
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.Em lazy
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transaction, which is allowed while suspending the file system, but
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blocked while the file system is suspended.
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.Pp
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Transactions are per-thread and nestable: if a thread is already in a
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transaction, it can enter another transaction without blocking.
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Each
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.Fn fstrans_start
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must be paired with
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.Fn fstrans_done .
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Transactions for multiple distinct mount points may not be nested.
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.Pp
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The file system's
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.Xr VFS_SUSPENDCTL 9
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method can use
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.Fn fstrans_setstate
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to:
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.Bl -dash
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.It
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enter the
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.Dv FSTRANS_SUSPENDING
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state to suspend all normal operations but allow lazy transactions,
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.It
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enter the
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.Dv FSTRANS_SUSPENDED
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state to suspend all operations, and
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.It
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restore to the
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.Dv FSTRANS_NORMAL
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state to resume all operations.
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.El
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.Pp
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A file system supporting
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.Nm
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may establish a copy-on-write callback with
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.Fn fscow_establish .
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The copy-on-write callback will be called every time a buffer is
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written to a block device with
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.Fn VOP_STRATEGY
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and every time a buffer is read into the
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.Xr buffercache 9
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with
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.Dv B_MODIFY
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set indicating the caller's intent to modify it.
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Anyone modifying a buffer may additionally use
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.Fn fscow_run
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to explicitly invoke the established callback.
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The copy-on-write callback must be disestablished with
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.Fn fscow_disestablish
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when the file system is done with it.
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.Sh FUNCTIONS
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.Bl -tag -width abcd
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.It Fn fstrans_start "mp"
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Enter a transaction on the file system
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.Fa mp
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in the current thread.
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If the file system is in a state that blocks such transactions, wait
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until it changes state to one that does not.
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.Pp
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If the file system is suspended, wait until it is resumed.
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.Pp
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However, if the current thread is already in a transaction on
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.Fa mp ,
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.Fn fstrans_start
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will enter a nested transaction and return immediately without
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waiting.
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.Pp
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May sleep.
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.It Fn fstrans_start_nowait "mp"
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Like
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.Fn fstrans_start ,
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but return
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.Er EBUSY
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immediately if transactions are blocked in its current state.
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.Pp
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May sleep nevertheless on internal locks.
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.It Fn fstrans_start_lazy "mp"
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Like
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.Fn fstrans_start ,
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but will not block while suspending.
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.Pp
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May sleep.
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.It Fn fstrans_done "mp"
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End the current transaction on
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.Fa mp .
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.It Fn fstrans_getstate "mp"
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Return the current state of the file system
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.Fa mp .
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.Pp
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Must be called within a transaction for the answer to be stable.
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.It Fn fstrans_setstate "mp" "new_state"
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Change the state of the file system
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.Fa mp
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to
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.Fa new_state ,
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and wait for all transactions not allowed in
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.Fa new_state
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to complete.
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.Bl -tag -width "Dv FSTRANS_SUSPENDING"
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.It Dv FSTRANS_NORMAL
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Allow all transactions.
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.It Dv FSTRANS_SUSPENDING
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Block
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.Dv FSTRANS_SHARED
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transactions but allow
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.Dv FSTRANS_LAZY
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transactions.
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.It Dv FSTRANS_SUSPENDED
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Block all transactions.
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.El
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.Pp
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A thread that changes a file system to a state other than
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.Dv FSTRANS_NORMAL
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enters a transaction for the purposes of
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.Fn fstrans_getstate
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until it changes state back to
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.Dv FSTRANS_NORMAL .
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.Pp
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Additionally, a thread that changes a file system to a state other than
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.Dv FSTRANS_NORMAL
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acquires an exclusive lock on the file system state, so that
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.Fn fstrans_is_owner
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will return true in that thread, and no other thread can change the
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file system's state until the owner restores it to
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.Dv FSTRANS_NORMAL .
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.Pp
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May sleep, and may be interrupted by a signal.
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On success, return zero.
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On failure, restore the file system to the
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.Dv FSTRANS_NORMAL
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state and return an error code.
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.Fn fstrans_setstate
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never fails if
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.Fa new_state
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is
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.Dv FSTRANS_NORMAL .
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.It Fn fstrans_is_owner "mp"
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Return
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.Dv true
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if the current thread is currently suspending the file system
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.Fa mp .
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.It Fn fscow_establish "mp" "func" "cookie"
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Establish a copy-on-write callback for the file system
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.Fa mp .
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The function
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.Fa func
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will be called for every buffer
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.Fa bp
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written through this file system as
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.Dl Fa func Ns ( Fa cookie , Fa bp , Fa data_valid )
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where
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.Fa data_valid
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is true if and only if the buffer
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.Fa bp
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has not yet been modified.
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.Pp
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May sleep.
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.It Fn fscow_disestablish "mp" "func" "cookie"
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Disestablish a copy-on-write callback established with
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.Fn fscow_establish .
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.Pp
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May sleep.
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.It Fn fscow_run "bp" "data_valid"
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Run all copy-on-write callbacks established for the file system this buffer
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belongs to, if they have not already been run for this buffer.
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If
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.Fa data_valid
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is
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.Dv true
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the buffer data has not yet been modified.
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.Pp
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May sleep.
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.El
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.Sh EXAMPLES
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The following is an example of a file system suspend operation.
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.Bd -literal
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int
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xxx_suspendctl(struct mount *mp, int cmd)
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{
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int error;
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switch (cmd) {
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case SUSPEND_SUSPEND:
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error = fstrans_setstate(mp, FSTRANS_SUSPENDING);
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if (error)
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return error;
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return fstrans_setstate(mp, FSTRANS_SUSPENDED);
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case SUSPEND_RESUME:
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return fstrans_setstate(mp, FSTRANS_NORMAL);
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default:
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return EINVAL;
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}
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}
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.Ed
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.Pp
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This is an example of a file system operation.
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.Bd -literal
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int
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xxx_create(void *v)
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{
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struct vop_create_args *ap = v;
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struct mount *mp = ap->a_dvp->v_mount;
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int error;
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fstrans_start(mp);
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/* Actually create the node. */
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fstrans_done(mp);
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return 0;
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}
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.Ed
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.Sh CODE REFERENCES
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The
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.Nm
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subsystem is implemented in the file
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.Pa sys/kern/vfs_trans.c .
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.Sh SEE ALSO
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.Xr vfs_resume 9 ,
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.Xr vfs_suspend 9
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.Sh HISTORY
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The
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.Nm
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subsystem appeared in
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.Nx 5.0 .
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.Sh AUTHORS
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The
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.Nm
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subsystem was written by
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.An J\(:urgen Hannken-Illjes
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.Aq hannken@NetBSD.org .
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.Sh BUGS
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.Nm
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is useful only for temporary suspension \(em it does not help to
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permanently block file system operations for unmounting, because
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.Fn fstrans_start
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cannot fail.
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