2000-10-24 03:33:24 +00:00
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.\" Copyright (c) 2000
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" 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, 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 DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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2000-11-22 13:31:01 +00:00
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.Dd October 24, 2000
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2000-10-24 03:33:24 +00:00
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.Dt LOCK 9
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.Os FreeBSD
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.Sh NAME
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.Nm lockinit ,
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.Nm lockmgr ,
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.Nm lockcount ,
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.Nm lockstatus
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.Nd kernel process locking
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <sys/lock.h>
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.Ft void
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.Fn lockinit "struct lock *lkp" "int prio" "char *wmesg" "int timo" "int flags"
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.Ft int
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.Fn lockmgr "struct lock *lkp" "u_int flags" "struct simplelock *interlkp" "struct proc *p"
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.Ft int
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.Fn lockcount "struct lock *lkp"
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.Ft int
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.Fn lockstatus "struct lock *lkp" "struct proc *p"
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.Sh DESCRIPTION
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.Pp
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The function
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.Fn lockinit
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is used to initialise a lock.
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It is required if locks are used.
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The function
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.Fn lockmgr
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is used to manage locks.
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The function
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.Fn lockcount
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returns the number of shared lock holders on a lock.
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The function
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.Fn lockstatus
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determines the status of a lock.
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.Pp
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The following values are defined:
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.Bl -tag -width "LK_XXXXXXXXXXXX" -compact
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.It Dv LK_SHARED
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get one of many possible shared locks.
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If a process holding an exclusive lock requests a shared lock,
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the exclusive lock(s) will be downgraded to shared locks.
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.It Dv LK_EXCLUSIVE
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stop further shared locks, when they are cleared,
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grant a pending upgrade if it exists, then grant an exclusive
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lock.
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Only one exclusive lock may exist at a time, except that
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a process holding an exclusive lock may get additional exclusive
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locks if it explicitly sets the LK_CANRECURSE flag in the lock
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request, or if the LK_CANRECUSE flag was set when the lock was
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initialized.
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.It Dv LK_UPGRADE
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the process must hold a shared lock that it wants to
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have upgraded to an exclusive lock.
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Other processes may get exclusive access to the resource between
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the time that the upgrade is requested and the time that it is
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granted.
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.It Dv LK_EXCLUPGRADE
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the process must hold a shared lock that it wants to
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have upgraded to an exclusive lock.
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If the request succeeds, no other processes will have gotten
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exclusive access to the resource between the time that the upgrade
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is requested and the time that it is granted.
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However, if another process has already requested an upgrade,
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the request will fail.
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.It Dv LK_DOWNGRADE
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the process must hold an exclusive lock that it wants
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to have downgraded to a shared lock.
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If the process holds multiple (recursive) exclusive locks,
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they will all be downgraded to shared locks.
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.It Dv LK_RELEASE
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release one instance of a lock.
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.It Dv LK_DRAIN
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wait for all activity on the lock to end, then mark it
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decommissioned.
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This feature is used before freeing a lock that is part of a
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piece of memory that is about to be freed.
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.It Dv LK_EXCLOTHER
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return for lockstatus().
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Used when another process holds the lock exclusively.
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.El
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.Sh RETURN VALUES
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Successfully obtained locks return 0.
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Locks will always succeed unless one of the following is true:
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LK_FORCEUPGRADE is requested and some other process has already
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requested a lock upgrade; returns
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.Er EBUSY .
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LK_WAIT is set and a sleep would be required; returns
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.Er EBUSY .
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LK_SLEEPFAIL is set and a sleep was done; returns
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.Er ENOLCK .
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PCATCH is set in lock priority and a signal arrives; returns
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either
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.Er EINTR
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or
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.Er ERESTART
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if system calls is to be restarted.
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Non-null lock timeout and timeout expires; returns
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.Er EWOULDBLOCK .
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A failed lock attempt always returns a non-zero error value.
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No lock is held after an error return (in particular, a failed
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LK_UPGRADE or LK_FORCEUPGRADE will have released its shared
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access lock).
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