641a1aa10d
when using the callout subsystem. Show how the callout_pending(), callout_active() and callout_deactivate() macros can be used to achieve simpler race-free callout semantics in many situations.
476 lines
13 KiB
Groff
476 lines
13 KiB
Groff
.\" $NetBSD: timeout.9,v 1.2 1996/06/23 22:32:34 pk Exp $
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.\"
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.\" Copyright (c) 1996 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 Paul Kranenburg.
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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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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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 REGENTS OR CONTRIBUTORS BE
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.\" 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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.\" $FreeBSD$
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.\"
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.Dd January 23, 2005
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.Dt TIMEOUT 9
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.Os
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.Sh NAME
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.Nm timeout ,
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.Nm untimeout ,
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.Nm callout_handle_init ,
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.Nm callout_init ,
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.Nm callout_stop ,
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.Nm callout_drain ,
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.Nm callout_reset ,
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.Nm callout_pending ,
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.Nm callout_active ,
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.Nm callout_deactivate
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.Nd execute a function after a specified length of time
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/systm.h
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.Pp
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.Bd -literal
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typedef void timeout_t (void *);
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.Ed
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.Ft struct callout_handle
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.Fn timeout "timeout_t *func" "void *arg" "int ticks"
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.Ft void
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.Fn callout_handle_init "struct callout_handle *handle"
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.Pp
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.Bd -literal
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struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle)
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.Ed
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.Ft void
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.Fn untimeout "timeout_t *func" "void *arg" "struct callout_handle handle"
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.Ft void
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.Fn callout_init "struct callout *c" "int mpsafe"
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.Ft int
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.Fn callout_stop "struct callout *c"
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.Ft int
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.Fn callout_drain "struct callout *c"
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.Ft void
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.Fn callout_reset "struct callout *c" "int ticks" "timeout_t *func" "void *arg"
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.Ft int
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.Fn callout_pending "struct callout *c"
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.Ft int
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.Fn callout_active "struct callout *c"
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.Fn callout_deactivate "struct callout *c"
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.Sh DESCRIPTION
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The function
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.Fn timeout
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schedules a call to the function given by the argument
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.Fa func
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to take place after
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.Fa ticks Ns No /hz
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seconds.
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Non-positive values of
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.Fa ticks
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are silently converted to the value
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.Sq 1 .
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.Fa func
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should be a pointer to a function that takes a
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.Fa void *
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argument.
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Upon invocation,
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.Fa func
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will receive
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.Fa arg
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as its only argument.
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The return value from
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.Fn timeout
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is a
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.Ft struct callout_handle
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which can be used in conjunction with the
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.Fn untimeout
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function to request that a scheduled timeout be canceled.
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The
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.Fn timeout
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call is the old style and new code should use the
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.Fn callout_*
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functions.
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.Pp
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The function
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.Fn callout_handle_init
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can be used to initialize a handle to a state which will cause
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any calls to untimeout with that handle to return with no side
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effects.
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.Pp
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Assigning a callout handle the value of
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.Fn CALLOUT_HANDLE_INITIALIZER
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performs the same function as
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.Fn callout_handle_init
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and is provided for use on statically declared or global callout handles.
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.Pp
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The function
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.Fn untimeout
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cancels the timeout associated with
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.Fa handle
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using the
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.Fa func
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and
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.Fa arg
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arguments to validate the handle.
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If the handle does not correspond to a timeout with
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the function
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.Fa func
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taking the argument
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.Fa arg
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no action is taken.
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.Fa handle
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must be initialized by a previous call to
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.Fn timeout ,
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.Fn callout_handle_init ,
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or assigned the value of
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.Fn CALLOUT_HANDLE_INITIALIZER "&handle"
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before being passed to
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.Fn untimeout .
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The behavior of calling untimeout without a previously initialized handle
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is undefined.
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The
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.Fn untimeout
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call is the old style and new code should use the
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.Fn callout_*
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functions.
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.Pp
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As handles are recycled by the system, it is possible (although unlikely)
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that a handle from one invocation of
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.Fn timeout
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may match the handle of another invocation of
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.Fn timeout
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if both calls used the same function pointer and argument, and the first
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timeout is expired or canceled before the second call.
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The timeout facility offers O(1) running time for
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.Fn timeout
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and
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.Fn untimeout .
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Timeouts are executed from
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.Fn softclock
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with the
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.Va Giant
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lock held.
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Thus they are protected from re-entrancy.
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.Pp
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The functions
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.Fn callout_init ,
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.Fn callout_stop ,
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.Fn callout_drain
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and
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.Fn callout_reset
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are low-level routines for clients who wish to allocate their own
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callout structures.
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.Pp
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The function
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.Fn callout_init
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initializes a callout so it can be passed to
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.Fn callout_stop ,
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.Fn callout_drain
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or
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.Fn callout_reset
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without any side effects.
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If the
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.Fa mpsafe
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argument is zero,
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the callout structure is not considered to be
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.Dq multi-processor safe ;
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that is,
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the Giant lock will be acquired before calling the callout function,
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and released when the callout function returns.
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.Pp
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The function
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.Fn callout_stop
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cancels a callout if it is currently pending.
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If the callout is pending, then
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.Fn callout_stop
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will return a non-zero value.
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If the callout is not set, has already been serviced or is currently
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being serviced, then zero will be returned.
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.Pp
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The function
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.Fn callout_drain
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is identical to
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.Fn callout_stop
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except that it will wait for the callout to be completed if it is
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already in progress.
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This function MUST NOT be called while holding any
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locks on which the callout might block, or deadlock will result.
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Note that if the callout subsystem has already begun processing this
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callout, then the callout function may be invoked during the execution of
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.Fn callout_drain .
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However, the callout subsystem does guarantee that the callout will be
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fully stopped before
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.Fn callout_drain
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returns.
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.Pp
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The function
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.Fn callout_reset
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first first performs the equivalent of
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.Fn callout_stop
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to disestablish the callout, and then establishes a new callout in the
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same manner as
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.Fn timeout .
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.Pp
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The macros
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.Fn callout_pending ,
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.Fn callout_active
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and
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.Fn callout_deactivate
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provide access to the current state of the callout.
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Careful use of these macros can avoid many of the race conditions
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that are inherent in asynchronous timer facilities; see
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.Sx "Avoiding Race Conditions"
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below for further details.
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The
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.Fn callout_pending
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macro checks whether a callout is
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.Em pending ;
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a callout is considered
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.Em pending
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when a timeout has been set but the time has not yet arrived.
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Note that once the timeout time arrives and the callout subsystem
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starts to process this callout,
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.Fn callout_pending
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will return
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.Dv FALSE
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even though the callout function may not have finished (or even begun)
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executing.
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The
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.Fn callout_active
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macro checks whether a callout is marked as
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.Em active ,
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and the
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.Fn callout_deactivate
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macro clears the callout's
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.Em active
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flag.
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The callout subsystem marks a callout as
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.Em active
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when a timeout is set and it clears the
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.Em active
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flag in
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.Fn callout_stop
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and
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.Fn callout_drain ,
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but it
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.Em does not
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clear it when a callout expires normally via the execution of the
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callout function.
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.Ss "Avoiding Race Conditions"
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The callout subsystem invokes callout functions from its own timer
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context.
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Without some kind of synchronization it is possible that a callout
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function will be invoked concurrently with an attempt to stop or reset
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the callout by another thread.
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In particular, since callout functions typically acquire a mutex as
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their first action, the callout function may have already been invoked,
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but be blocked waiting for that mutex at the time that another thread
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tries to reset or stop the callout.
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.Pp
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The callout subsystem provides a number of mechanisms to address these
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synchronization concerns:
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.Bl -enum -offset indent -compact
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.It
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The return value from
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.Fn callout_stop
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indicates whether or not the callout was removed.
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If it is known that the callout was set and the callout function has
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not yet executed, then a return value of
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.Dv FALSE
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indicates that the callout function is about to be called.
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For example:
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.Bd -literal -offset indent
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if (sc->sc_flags & SCFLG_CALLOUT_RUNNING) {
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if (callout_stop(&sc->sc_callout)) {
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sc->sc_flags &= ~SCFLG_CALLOUT_RUNNING;
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/* successfully stopped */
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} else {
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/*
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* callout has expired and callout
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* function is about to be executed
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*/
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}
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}
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.Ed
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.Pp
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Note that there is no equivalent mechanism to determine whether or not
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.Fn callout_reset
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stopped the callout.
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.It
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The
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.Fn callout_pending ,
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.Fn callout_active
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and
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.Fn callout_deactivate
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macros can be used together to work around the race conditions.
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When a callout's timeout is set, the callout subsystem marks the
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callout as both
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.Em active
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and
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.Em pending .
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When the timeout time arrives, the callout subsystem begins processing
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the callout by first clearing the
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.Em pending
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flag.
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It then invokes the callout function without changing the
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.Em active
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flag, and does not clear the
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.Em active
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flag even after the callout function returns.
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The mechanism described here requires the callout function itself to
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clear the
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.Em active
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flag using the
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.Fn callout_deactivate
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macro.
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The
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.Fn callout_stop
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and
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.Fn callout_drain
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functions always clear both the
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.Em active
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and
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.Em pending
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flags before returning.
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.Pp
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The callout function should first check the
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.Em pending
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flag and return without action if
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.Fn callout_pending
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returns
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.Dv TRUE .
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This indicates that the callout was rescheduled using
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.Fn callout_reset
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just before the callout function was invoked.
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If
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.Fn callout_active
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returns
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.Dv FALSE
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then the callout function should also return without action.
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This indicates that the callout has been stopped.
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Finally, the callout function should call
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.Fn callout_deactivate
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to clear the
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.Em active
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flag.
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For example:
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.Bd -literal -offset indent
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mtx_lock(&sc->sc_mtx);
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if (callout_pending(&sc->sc_callout)) {
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/* callout was reset */
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mtx_unlock(&sc->sc_mtx);
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return;
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}
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if (!callout_active(&sc->sc_callout)) {
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/* callout was stopped */
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mtx_unlock(&sc->sc_mtx);
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return;
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}
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callout_deactivate(&sc->sc_callout);
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/* rest of callout function */
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.Ed
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.Pp
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Together with appropriate synchronization, such as the mutex used above,
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this approach permits the
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.Fn callout_stop
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and
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.Fn callout_reset
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functions to be used at any time without races.
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For example:
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.Bd -literal -offset indent
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mtx_lock(&sc->sc_mtx);
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callout_stop(&sc->sc_callout);
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/* The callout is effectively stopped now. */
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.Ed
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.Pp
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If the callout is still pending then these functions operate normally,
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but if processing of the callout has already begun then the tests in
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the callout function cause it to return without further action.
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Synchronization between the callout function and other code ensures that
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stopping or resetting the callout will never be attempted while the
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callout function is past the
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.Fn callout_deactivate
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call.
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.Pp
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The above technique additionally ensures that the
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.Em active
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flag always reflects whether the callout is effectively enabled or
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disabled.
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If
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.Fn callout_active
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returns false, then the callout is effectively disabled, since even if
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the callout subsystem is actually just about to invoke the callout
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function, the callout function will return without action.
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.El
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.Pp
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There is one final race condition that must be considered when a
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callout is being stopped for the last time.
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In this case it may not be safe to let the callout function itself
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detect that the callout was stopped, since it may need to access
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data objects that have already been destroyed or recycled.
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To ensure that the callout is completely finished, a call to
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.Fn callout_drain
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should be used.
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.Sh RETURN VALUES
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The
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.Fn timeout
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function returns a
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.Ft struct callout_handle
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that can be passed to
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.Fn untimeout .
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The
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.Fn callout_stop
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and
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.Fn callout_drain
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functions return non-zero if the callout was still pending when it was
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called or zero otherwise.
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.Sh HISTORY
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The current timeout and untimeout routines are based on the work of
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.An Adam M. Costello
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and
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.An George Varghese ,
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published in a technical report entitled
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.%T "Redesigning the BSD Callout and Timer Facilities"
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and modified slightly for inclusion in
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.Fx
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by
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.An Justin T. Gibbs .
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The original work on the data structures used in this implementation
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was published by
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.An G. Varghese
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and
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.An A. Lauck
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in the paper
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.%T "Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient Implementation of a Timer Facility"
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in the
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.%B "Proceedings of the 11th ACM Annual Symposium on Operating Systems Principles" .
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The current implementation replaces the long standing
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.Bx
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linked list
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callout mechanism which offered O(n) insertion and removal running time
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but did not generate or require handles for untimeout operations.
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