217 lines
6.6 KiB
Groff
217 lines
6.6 KiB
Groff
.\"
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.\" Copyright (c) 1996 Joerg Wunsch
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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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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $Id: sleep.9,v 1.11 1998/12/21 10:29:28 dillon Exp $
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.\" "
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.Dd December 17, 1998
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.Os
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.Dt SLEEP 9
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.Sh NAME
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.Nm sleep ,
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.Nm tsleep ,
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.Nm asleep ,
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.Nm await ,
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.Nm wakeup
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.Nd wait for events
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.Sh SYNOPSIS
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.Fd #include <sys/param.h>
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.Fd #include <sys/systm.h>
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.Fd #include <sys/proc.h>
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.Ft int
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.Fn tsleep "void *ident" "int priority" "const char *wmesg" "int timo"
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.Ft int
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.Fn asleep "void *ident" "int priority" "const char *wmesg" "int timo"
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.Ft int
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.Fn await "int priority" "int timo"
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.Ft void
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.Fn wakeup "void *ident"
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.Ft void
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.Fn wakeup_one "void *ident"
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.Sh DESCRIPTION
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The functions
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.Fn tsleep
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and
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.Fn wakeup
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handle event-based process blocking. If a process must wait for an
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external event, it is put on sleep by
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.Nm tsleep .
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The parameter
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.Ar ident
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is an arbitrary address that uniquely identifies the event on which
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the process is being asleep. All processes sleeping on a single
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.Ar ident
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are woken up later by
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.Nm wakeup ,
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often called from inside an interrupt routine, to indicate that the
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resource the process was blocking on is available now.
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.Pp
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The parameter
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.Ar wmesg
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is a string describing the sleep condition for tools like
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.Xr ps 1 .
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Due to the limited space of those programs to display arbitrary strings,
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this message should not be longer than 6 characters.
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.Pp
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The
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.Fn wakeup_one
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function is used to make the first process in the queue that is
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sleeping on the parameter
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.Fa ident
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runnable. This can prevent the system from becoming saturated
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when a large number of processes are sleeping on the same address,
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but only one of them can actually do any useful work when made
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runnable.
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.Pp
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.Nm Tsleep
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is the general sleep call. Suspends the current process until a wakeup is
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performed on the specified identifier. The process will then be made
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runnable with the specified
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.Ar priority .
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Sleeps at most
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.Ar timo
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\&/ hz seconds (0 means no timeout). If
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.Ar pri
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includes the
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.Dv PCATCH
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flag, signals are checked before and after sleeping, else signals are
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not checked. Returns 0 if awakened,
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.Dv EWOULDBLOCK
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if the timeout expires. If
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.Dv PCATCH
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is set and a signal needs to be delivered,
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.Dv ERESTART
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is returned if the current system call should be restarted if
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possible, and
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.Dv EINTR
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is returned if the system call should be interrupted by the signal
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.Pq return Dv EINTR .
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.Pp
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.Nm Sleep
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is the traditional form. It doesn't let you specify a timeout nor a
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.Ar wmesg ,
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hence its use is deprecated.
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.Pp
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.Nm Asleep
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implements the new asynchronous sleep function. It takes the same arguments
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as
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.Fn tsleep
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and places the process on the appropriate wait queue, but
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.Fn asleep
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leaves the process runnable and returns immediately. The caller is then
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expected to, at some point in the future, call
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.Fn await
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to actually wait for the previously queued wait condition.
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If
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.Fn asleep
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is called several times, only the most recent call is effective.
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.Fn asleep
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may be called with an
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.Ar ident
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value of NULL
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to remove any previously queued condition.
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.Pp
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.Nm Await
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implements the new asynchronous wait function. If you
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.Fn asleep
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on an identifier,
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.Fn await
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will actually block the process until someone calls
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.Fn wakeup
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on that identifier. If someone calls
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.Fn wakeup
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after you
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.Fn asleep
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but before you
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.Fn await
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then the
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.Fn await
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call is effectively a NOP.
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If
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.Fn await
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is called multiple times without an intervening
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.Fn asleep
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the
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.Fn await
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is effective a NOP, but will call
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.Fn mswitch
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for safety. The
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.Fn await
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function allows you to override the priority and timeout values to be used.
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If the value -1 is specified for an argument, the value is taken from the
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previous
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.Fn asleep
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call. If you pass -1 for the priority you must be prepared to catch signal
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conditions if the prior call to
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.Fn asleep
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specified it in its priority. If you pass -1 for the timeout you must be
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prepared to catch a timeout condition if the prior call to
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.Fn asleep
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specified a timeout. When you use -1, you should generally not make
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assumptions as to the arguments used by the prior
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.Fn asleep
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call.
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.Pp
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The
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.Fn asleep
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and
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.Fn await
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functions are used by the kernel code for various purposes but the main one is
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to allow complex interlocking code to 'backout' of a temporary resource failure
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(such as lack of memory or trying to access a block that is not in the buffer
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cache) in order to release major locks prior to blocking, and to then retry
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the call that failed on wakeup. This involves subroutines deep in the kernel
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calling
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.Fn asleep
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and returning a temporary failure, then popping back up through a number
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of call levels before calling
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.Fn await ,
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then retrying. The kernel might also use these functions to avoid using
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spinlocks in a check-condition interlock. That is, in case the case where
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the kernel wishes to check the condition of something and then block on it.
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To avoid the race between the check and the blocking, the kernel can first
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check the condition, then call
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.Fn asleep ,
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then check the condition a second time before calling
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.Fn await .
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The overlap makes the race condition impossible.
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.Sh RETURN VALUES
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See above.
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.Sh SEE ALSO
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.Xr ps 1
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.Sh HISTORY
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The sleep/wakeup process synchronization mechanism is very old. It
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appeared in a very early version of Unix.
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.Pp
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.Nm Tsleep
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appeared in
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.Bx 4.4 .
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.Pp
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.Nm Asleep/await
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first appeared in FreeBSD-3.0.1
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.Sh AUTHORS
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This man page has been written by
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.ie t J\(:org Wunsch.
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.el Joerg Wunsch. asleep/await portions were written by Matt Dillon
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