285 lines
7.3 KiB
Groff
285 lines
7.3 KiB
Groff
.\"
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.\" Copyright (C) 2002 Chad David <davidc@acns.ab.ca>. All rights reserved.
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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(s), this list of conditions and the following disclaimer as
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.\" the first lines of this file unmodified other than the possible
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.\" addition of one or more copyright notices.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
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.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
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.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
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.\" DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 6, 2008
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.Dt LOCK 9
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.Os
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.Sh NAME
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.Nm lockinit ,
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.Nm lockdestroy ,
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.Nm lockmgr ,
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.Nm lockstatus ,
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.Nm lockmgr_disown ,
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.Nm lockmgr_printinfo ,
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.Nm lockmgr_recursed
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.Nd "lockmgr family of functions"
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/lockmgr.h
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.Ft void
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.Fn lockinit "struct lock *lkp" "int prio" "const char *wmesg" "int timo" "int flags"
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.Ft void
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.Fn lockdestroy "struct lock *lkp"
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.Ft int
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.Fn lockmgr "struct lock *lkp" "u_int flags" "struct mtx *interlkp"
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.Ft int
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.Fn lockstatus "struct lock *lkp" "struct thread *td"
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.Ft void
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.Fn lockmgr_disown "struct lock *lkp"
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.Ft void
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.Fn lockmgr_printinfo "struct lock *lkp"
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.Ft int
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.Fn lockmgr_recursed "struct lock *lkp"
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.Sh DESCRIPTION
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The
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.Fn lockinit
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function is used to initialize a lock.
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It must be called before any operation can be performed on a lock.
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Its arguments are:
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.Bl -tag -width ".Fa wmesg"
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.It Fa lkp
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A pointer to the lock to initialize.
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.It Fa prio
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The priority passed to
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.Xr sleep 9 .
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.It Fa wmesg
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The lock message.
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This is used for both debugging output and
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.Xr sleep 9 .
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.It Fa timo
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The timeout value passed to
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.Xr sleep 9 .
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.It Fa flags
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The flags the lock is to be initialized with.
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.Bl -tag -width ".Dv LK_CANRECURSE"
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.It Dv LK_NOWAIT
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Do not sleep while acquiring the lock.
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.It Dv LK_SLEEPFAIL
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Fail after a sleep.
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.It Dv LK_CANRECURSE
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Allow recursive exclusive locks.
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.It Dv LK_NOSHARE
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Allow exclusive locks only.
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.It Dv LK_NOWITNESS
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Instruct
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.Xr witness 4
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to ignore this lock.
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.It Dv LK_NODUP
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.Xr witness 4
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should log messages about duplicate locks being acquired.
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.It Dv LK_TIMELOCK
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Use
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.Fa timo
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during a sleep; otherwise, 0 is used.
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.El
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.El
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.Pp
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The
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.Fn lockdestroy
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function is used to destroy a lock, and while it is called in a number of
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places in the kernel, it currently does nothing.
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.Pp
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The
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.Fn lockmgr
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function handles general locking functionality within the kernel, including
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support for shared and exclusive locks, and recursion.
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.Fn lockmgr
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is also able to upgrade and downgrade locks.
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.Pp
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Its arguments are:
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.Bl -tag -width ".Fa interlkp"
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.It Fa lkp
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A pointer to the lock to manipulate.
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.It Fa flags
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Flags indicating what action is to be taken.
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.Bl -tag -width ".Dv LK_CANRECURSE"
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.It Dv LK_SHARED
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Acquire a shared lock.
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If an exclusive lock is currently held, it will be downgraded.
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.It Dv LK_EXCLUSIVE
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Acquire an exclusive lock.
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If an exclusive lock is already held, and
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.Dv LK_CANRECURSE
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is not set, the system will
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.Xr panic 9 .
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.It Dv LK_DOWNGRADE
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Downgrade exclusive lock to a shared lock.
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Downgrading a shared lock is not permitted.
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If an exclusive lock has been recursed, all references will be downgraded.
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.It Dv LK_UPGRADE
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Upgrade a shared lock to an exclusive lock.
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If this call fails, the shared lock is lost.
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During the upgrade, the shared lock could
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be temporarily dropped.
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Attempts to upgrade an exclusive lock will cause a
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.Xr panic 9 .
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.It Dv LK_RELEASE
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Release the lock.
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Releasing a lock that is not held can cause a
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.Xr panic 9 .
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.It Dv LK_DRAIN
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Wait for all activity on the lock to end, then mark it decommissioned.
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This is used before freeing a lock that is part of a piece of memory that is
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about to be freed.
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(As documented in
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.In sys/lockmgr.h . )
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.It Dv LK_SLEEPFAIL
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Fail if operation has slept.
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.It Dv LK_NOWAIT
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Do not allow the call to sleep.
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This can be used to test the lock.
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.It Dv LK_CANRECURSE
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Allow recursion on an exclusive lock.
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For every lock there must be a release.
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.It Dv LK_INTERLOCK
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Unlock the interlock (which should be locked already).
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.El
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.It Fa interlkp
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An interlock mutex for controlling group access to the lock.
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If
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.Dv LK_INTERLOCK
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is specified,
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.Fn lockmgr
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assumes
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.Fa interlkp
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is currently owned and not recursed, and will return it unlocked.
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See
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.Xr mtx_assert 9 .
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.El
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.Pp
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The
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.Fn lockstatus
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function returns the status of the lock in relation to the
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.Vt thread
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passed to it.
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Note that if
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.Fa td
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is
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.Dv NULL
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and an exclusive lock is held,
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.Dv LK_EXCLUSIVE
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will be returned.
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.Pp
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The
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.Fn lockmgr_disown
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function switches the owner from curthread to be
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.Dv LK_KERNPROC,
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if the lock is already held.
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.Pp
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The
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.Fn lockmgr_printinfo
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function prints debugging information about the lock.
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It is used primarily by
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.Xr VOP_PRINT 9
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functions.
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.Pp
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The
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.Fn lockmgr_recursed
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function returns true if the lock is recursed, 0
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otherwise.
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.Sh RETURN VALUES
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The
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.Fn lockmgr
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function returns 0 on success and non-zero on failure.
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.Pp
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The
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.Fn lockstatus
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function returns:
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.Bl -tag -width ".Dv LK_EXCLUSIVE"
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.It Dv LK_EXCLUSIVE
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An exclusive lock is held by the thread
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.Fa td .
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.It Dv LK_EXCLOTHER
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An exclusive lock is held by someone other than the thread
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.Fa td .
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.It Dv LK_SHARED
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A shared lock is held.
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.It Li 0
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The lock is not held by anyone.
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.El
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.Sh ERRORS
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.Fn lockmgr
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fails if:
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.Bl -tag -width Er
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.It Bq Er EBUSY
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.Dv LK_FORCEUPGRADE
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was requested and another thread had already requested a lock upgrade.
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.It Bq Er EBUSY
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.Dv LK_NOWAIT
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was set, and a sleep would have been required.
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.It Bq Er ENOLCK
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.Dv LK_SLEEPFAIL
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was set and
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.Fn lockmgr
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did sleep.
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.It Bq Er EINTR
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.Dv PCATCH
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was set in the lock priority, and a signal was delivered during a sleep.
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Note the
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.Er ERESTART
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error below.
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.It Bq Er ERESTART
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.Dv PCATCH
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was set in the lock priority, a signal was delivered during a sleep,
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and the system call is to be restarted.
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.It Bq Er EWOULDBLOCK
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a non-zero timeout was given, and the timeout expired.
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.El
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.Sh LOCKS
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If
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.Dv LK_INTERLOCK
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is passed in the
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.Fa flags
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argument to
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.Fn lockmgr ,
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the
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.Fa interlkp
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must be held prior to calling
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.Fn lockmgr ,
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and will be returned unlocked.
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.Pp
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Upgrade attempts that fail result in the loss of the lock that
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is currently held.
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Also, it is invalid to upgrade an
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exclusive lock, and a
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.Xr panic 9
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will be the result of trying.
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.Sh SEE ALSO
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.Xr condvar 9 ,
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.Xr locking 9 ,
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.Xr mutex 9 ,
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.Xr rwlock 9 ,
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.Xr sleep 9 ,
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.Xr sx 9 ,
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.Xr mtx_assert 9 ,
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.Xr panic 9 ,
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.Xr VOP_PRINT 9
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.Sh AUTHORS
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This manual page was written by
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.An Chad David Aq davidc@acns.ab.ca .
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