race illustration

test.html (text/html), 16.22 KB, created by Daniel Drake (RETIRED) on 2006-11-15 11:34:40 UTC

<font color=red>First process</font><br />
<font color=blue>2nd process</font><br /><br />

<font color=red>
rc (or whatever) invokes net.ath0 start<br />
we soon get to svc_start, and all of this code gets executed:<br />

<pre>
svc_start() {
	local x= y= retval=0 startinactive=

	# Do not try to start if i have done so already on runlevel change
	if is_runlevel_start && service_failed "${SVCNAME}" ; then
		return 1
	elif service_started "${SVCNAME}" ; then
		ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
		return 0
	elif service_inactive "${SVCNAME}" ; then
		if [[ ${IN_BACKGROUND} != "true" \
		&& ${IN_BACKGROUND} != "1" ]] ; then
			ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
			return 0
		fi
	elif [[ ${SOFTLEVEL} == "shutdown" || ${SOFTLEVEL} == "reboot" ]] ; then
		ewarn $"WARNING:  system shutting down, will not start" "${SVCNAME}"
		return 1
	elif [[ ${SOFTLEVEL} == "single" ]] ; then
		eerror $"ERROR:  system is in single user mode, will not start" "${SVCNAME}"
		return 1
	fi

	if ! <b>mark_service_starting "${SVCNAME}"</b> ; then
		if service_stopping "${SVCNAME}" ; then
			eerror $"ERROR:" " ${SVCNAME}" $"is already stopping."
		else
			eerror $"ERROR: "" ${SVCNAME}" $"is already starting."
		fi
		return 1
	fi

	if begin_service "${SVCNAME}" ; then
		svcbegin=true
	else
		svcbegin=false
	fi

	# Ensure that we clean up if we abort for any reason
	trap "svc_quit" INT QUIT TSTP

	# Store our e* messages in a buffer so we pretty print when parallel
	[[ ${RC_PARALLEL_STARTUP} == "yes" && ${RC_QUIET} != "yes" ]] \
		&& ebuffer "${svcdir}/ebuffer/${SVCNAME}"

	veinfo $"Service" "${SVCNAME}" $"starting"

	if [[ -n $(rc-depend --notrace -broken "${SVCNAME}") ]] ; then
		eerror $"ERROR:  Some services needed are missing.  Run"
		eerror "        ""'./${SVCNAME}" $"broken' for a list of those"
		eerror "        " $"services."  "${SVCNAME}" $"was not started."
		retval=1	
	fi

	# Save the IN_BACKGROUND var as we need to clear it for starting depends
	local ib_save="${IN_BACKGROUND}"
	unset IN_BACKGROUND

	if [[ ${retval} == "0" && ${RC_NO_DEPS} != "yes" ]] ; then
		# Start dependencies, if any.
		local startsvc=$(rc-depend -ineed -iuse "${SVCNAME}")
		if ! is_runlevel_start ; then
			for x in ${startsvc} ; do
				service_stopped "${x}" && start_service "${x}"
			done
		fi

		# Wait for dependencies to finish.
		local ineed=$(rc-depend --notrace -ineed "${SVCNAME}")
		for x in ${startsvc} $(rc-depend -iafter "${SVCNAME}") ; do
			local timeout=3
			while [[ ${timeout} -gt 0 ]] ; do
				service_started "${x}" && continue 2
				wait_service "${x}"
				service_started "${x}" && continue 2
				if service_inactive "${x}" || service_scheduled "${x}" ; then
					if [[ " ${startsvc} " == *" ${x} "* ||
						" ${ineed} " == *" $(rc-depend --notrace -iprovide "${x}") " ]] ; then
						svc_schedule_start "${x}" "${SVCNAME}"
						[[ -n ${startinactive} ]] && startinactive="${startinactive}, "
						startinactive="${startinactive}${x}"
					fi

					continue 2
				fi
				service_stopped "${x}" && break
	
				((timeout--))
			done

			[[ " ${ineed} " != *" ${x} "*  ]] && continue
		
			eerror "ERROR:" $"cannot start" "${SVCNAME}" $"as" "${x}" $"could not start"
			retval=1
			break
		done

		if [[ -n ${startinactive} && ${retval} == "0" ]] ; then
			# Change the last , to or for correct grammar.
			x="${startinactive##*, }"
			startinactive="${startinactive/%, ${x}/ or ${x}}"
			ewarn "WARNING:" " ${SVCNAME}" $"is scheduled to start when" "${startinactive}" $"has started."
			retval=1
		fi
	fi
		
	if [[ ${retval} == "0" ]] ; then
		IN_BACKGROUND="${ib_save}"
		veindent
		(
		[[ ${RC_QUIET} == "yes" ]] && RC_QUIET_STDOUT="yes"
		exit() {
			eerror $"DO NOT USE EXIT IN INIT.D SCRIPTS"
			eerror $"This IS a bug, please fix your broken init.d"
			unset -f exit
			exit "$@"
		}

		# Apply any ulimits if defined
		[[ -n ${RC_ULIMIT} ]] && ulimit ${RC_ULIMIT}
		
		start
		)
		retval="$?"
</pre>

net.ath0 is now in state 'starting'.<br /><br />

The above "start" call eventually causes execution to get to wpa_supplicant_pre_start():<br />

<pre>
wpa_supplicant_pre_start() {
	local iface="$1" opts= timeout= actfile= cfgfile=

	# We don't configure wireless if we're being called from
	# the background unless we're not currently running
	if ${IN_BACKGROUND} ; then
		if service_started_daemon "net.${iface}" /sbin/wpa_supplicant ; then
			if wpa_supplicant_exists "${iface}" ; then
				ESSID=$(wpa_supplicant_get_essid "${iface}")
				ESSIDVAR=$(bash_variable "${ESSID}")
				save_options "ESSID" "${ESSID}"
				metric=2000
			fi
			return 0
		fi
	fi

	save_options "ESSID" ""

	local ifvar=$(bash_variable "${iface}")
	opts="wpa_supplicant_${ifvar}"
	opts=" ${!opts} "
	
	# We only work on wirelesss interfaces unless a driver for wired
	# has been defined
	if [[ ${opts} != *" -Dwired "* && ${opts} != *" -D wired "* ]] ; then
		if ! wpa_supplicant_exists "${iface}" ; then
			veinfo $"wpa_supplicant only works on wireless" \
				"interfaces unless the -D wired option" \
				"is specified"
			return 0
		fi
	fi

	# If wireless-tools is installed, try and apply our user config
	# This is needed for some drivers - such as hostap because they start
	# the card in Master mode which causes problems with wpa_supplicant.
	if is_function iwconfig_defaults ; then
		if wpa_supplicant_exists "${iface}" ; then
			iwconfig_defaults "${iface}"
			iwconfig_user_config "${iface}"
		fi
	fi	

	# Check for rf_kill - only ipw supports this at present, but other
	# cards may in the future
	if [[ -e "/sys/class/net/${iface}/device/rf_kill" ]] ; then
		if [[ $( < "/sys/class/net/${iface}/device/rf_kill" ) != 0 ]] ; then
			ewarn $"Wireless radio has been killed for interface" "${iface}"
			local asc="associate_timeout_${ifvar}"
			if [[ -n ${!asc} && ${!asc} -ge 0 ]] ; then
				eerror $"As you have" "${asc}" $"set to 0 or" \
					$"greater we will abort as we cannot" \
					$"associate"
				return 1
			fi
			ewarn $"wpa_supplicant will launch, but not associate" \
				$"until wireles radio is re-enabled for" \
				$"interface" "${iface}"
		fi
	fi
	
	ebegin "Starting wpa_supplicant on" "${iface}"

	cfgfile="${opts##* -c}"
	if [[ -n ${cfgfile} && ${cfgfile} != "${opts}" ]] ; then
		[[ ${cfgfile:0:1} == " " ]] && cfgfile="${cfgfile# *}"
		cfgfile="${cfgfile%% *}"
	else
		# Support new and old style locations
		cfgfile="/etc/wpa_supplicant/wpa_supplicant-${iface}.conf"
		[[ ! -e ${cfgfile} ]] \
			&& cfgfile="/etc/wpa_supplicant/wpa_supplicant.conf"
		[[ ! -e ${cfgfile} ]] \
			&& cfgfile="/etc/wpa_supplicant.conf"
		opts="${opts} -c${cfgfile}"
	fi

	if [[ ! -f ${cfgfile} ]] ; then
		eend 1 "/etc/wpa_supplicant/wpa_supplicant.conf" $"not found!"
		return 1
	fi

	# Work out where the ctrl_interface dir is if it's not specified
	local ctrl_dir=$(sed -n -e 's/[ \t]*#.*//g;s/[ \t]*$//g;s/^ctrl_interface=//p' "${cfgfile}")
	if [[ -z ${ctrl_dir} ]] ; then
		ctrl_dir="${opts##* -C}"
		if [[ -n ${ctrl_dir} && ${ctrl_dir} != "${opts}" ]] ; then
			[[ ${ctrl_dir:0:1} == " " ]] && ctrl_dir="${ctrl_dir# *}"
			ctrl_dir="${ctrl_dir%% *}"
		else
			ctrl_dir="/var/run/wpa_supplicant"
			opts="${opts} -C${ctrl_dir}"
		fi
	fi
	save_options ctrl_dir "${ctrl_dir}"

	# Some drivers require the interface to be up
	interface_up "${iface}"

	# wpa_supplicant 0.4.0 and greater supports wpa_cli actions
	# This is very handy as if and when different association mechanisms are
	# introduced to wpa_supplicant we don't have to recode for them as
	# wpa_cli is now responsible for informing us of success/failure.
	# The downside of this is that we don't see the interface being configured
	# for DHCP/static.
	actfile="/etc/wpa_supplicant/wpa_cli.sh"
	opts="${opts} -W"

	eval start-stop-daemon --start --exec /sbin/wpa_supplicant \
		--pidfile "/var/run/wpa_supplicant-${iface}.pid" \
		-- "${opts}" -B -i"${iface}" \
		-P"/var/run/wpa_supplicant-${iface}.pid"
	eend "$?" || return 1

	# Starting wpa_supplication-0.4.0, we can get wpa_cli to
	# start/stop our scripts from wpa_supplicant messages
	<b>mark_service_inactive "net.${iface}"</b>
	ebegin $"Starting wpa_cli on" "${iface}"
	start-stop-daemon --start --exec /bin/wpa_cli \
		--pidfile "/var/run/wpa_cli-${iface}.pid" \
		-- -a"${actfile}" -p"${ctrl_dir}" -i"${iface}" \
		-P"/var/run/wpa_cli-${iface}.pid" -B
	eend "$?" || return 1
</pre>

At this point, wpa_supplicant and wpa_cli are running and net.ath0 is now in state INACTIVE.

When it's happy, wpa_supplicant starts scanning and associating. When it has associated, wpa_cli is notified, and wpa_cli launches /etc/wpa_supplicant/wpa_cli.sh<br /><br />

</font>
<font color=blue>
I'll call wpa_cli.sh the 2nd process. wpa_cli launches /etc/init.d/net.ath0 start and we eventually get back into svc_start():<br /><br />

[Note that the process 1 svc_start() hasnt finished yet, it's been preempted at the last mentioned point]<br />

<pre>
svc_start() {
	local x= y= retval=0 startinactive=

	# Do not try to start if i have done so already on runlevel change
	if is_runlevel_start && service_failed "${SVCNAME}" ; then
		return 1
	elif service_started "${SVCNAME}" ; then
		ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
		return 0
	elif service_inactive "${SVCNAME}" ; then
		if [[ ${IN_BACKGROUND} != "true" \
		&& ${IN_BACKGROUND} != "1" ]] ; then
			ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
			return 0
		fi
	elif [[ ${SOFTLEVEL} == "shutdown" || ${SOFTLEVEL} == "reboot" ]] ; then
		ewarn $"WARNING:  system shutting down, will not start" "${SVCNAME}"
		return 1
	elif [[ ${SOFTLEVEL} == "single" ]] ; then
		eerror $"ERROR:  system is in single user mode, will not start" "${SVCNAME}"
		return 1
	fi

	if ! <b>mark_service_starting "${SVCNAME}"</b> ; then
		if service_stopping "${SVCNAME}" ; then
			eerror $"ERROR:" " ${SVCNAME}" $"is already stopping."
		else
			eerror $"ERROR: "" ${SVCNAME}" $"is already starting."
		fi
		return 1
	fi
</pre>

net.ath0 is now in state 'starting'<br /><br />

</font>

<font color=red>
back to process 1. continuing where we left off, wpa_supplicant_assocaite gets called, which does nothing but "exit 0" and control is returned to svc_start():<br />

<pre>
		# Don't trust init scripts to reset indentation properly
		# Needed for ebuffer
		eoutdent 99999

		# If a service has been marked inactive, exit now as something
		# may attempt to start it again later
		if [[ ${retval} == "0" ]] && <b>service_inactive "${SVCNAME}" </b>; then
			svcinactive=0
			ewarn $"WARNING:" " ${SVCNAME}" $"has started but is inactive"
			if [[ ${RC_PARALLEL_STARTUP} == "yes" && ${RC_QUIET} != "yes" ]] ; then
				eflush
				ebuffer ""
			fi
			return 1
		fi
	fi
</pre>

the service_inactive check fails, so svc_start continues.

<pre>
	if [[ ${retval} != "0" ]] ; then
		if [[ ${svcinactive} == "0" ]] ; then
			mark_service_inactive "${SVCNAME}"
		elif [[ -z ${startinactive} ]] ; then
			mark_service_stopped "${SVCNAME}"
			is_runlevel_start && mark_service_failed "${SVCNAME}"
			eerror $"ERROR:" " ${SVCNAME}" $"failed to start"
		fi
	else
		svcstarted=0
		<b>mark_service_started "${SVCNAME}"</b>
		veinfo $"Service" "${SVCNAME}" $"started"
	fi
</pre>

net.ath0 is now in state STARTED even though we have no indication at all of the status of process 2 (this has not yet even thought about invoking dhcpcd).<br /><br /></font>

to conclude I will highlight in red the inner part of critical section in which the race is significant. in this area, svc_start can run again in another process, and alter the state of the process, and therefore alter the behavior of the first process outside of the highlighted section.<br /><br />

<pre>
svc_start() {
	local x= y= retval=0 startinactive=

	# Do not try to start if i have done so already on runlevel change
	if is_runlevel_start && service_failed "${SVCNAME}" ; then
		return 1
	elif service_started "${SVCNAME}" ; then
		ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
		return 0
	elif service_inactive "${SVCNAME}" ; then
		if [[ ${IN_BACKGROUND} != "true" \
		&& ${IN_BACKGROUND} != "1" ]] ; then
			ewarn $"WARNING:" " ${SVCNAME}" $"has already been started."
			return 0
		fi
	elif [[ ${SOFTLEVEL} == "shutdown" || ${SOFTLEVEL} == "reboot" ]] ; then
		ewarn $"WARNING:  system shutting down, will not start" "${SVCNAME}"
		return 1
	elif [[ ${SOFTLEVEL} == "single" ]] ; then
		eerror $"ERROR:  system is in single user mode, will not start" "${SVCNAME}"
		return 1
	fi

	if ! mark_service_starting "${SVCNAME}" ; then
		if service_stopping "${SVCNAME}" ; then
			eerror $"ERROR:" " ${SVCNAME}" $"is already stopping."
		else
			eerror $"ERROR: "" ${SVCNAME}" $"is already starting."
		fi
		return 1
	fi

	if begin_service "${SVCNAME}" ; then
		svcbegin=true
	else
		svcbegin=false
	fi

	# Ensure that we clean up if we abort for any reason
	trap "svc_quit" INT QUIT TSTP

	# Store our e* messages in a buffer so we pretty print when parallel
	[[ ${RC_PARALLEL_STARTUP} == "yes" && ${RC_QUIET} != "yes" ]] \
		&& ebuffer "${svcdir}/ebuffer/${SVCNAME}"

	veinfo $"Service" "${SVCNAME}" $"starting"

	if [[ -n $(rc-depend --notrace -broken "${SVCNAME}") ]] ; then
		eerror $"ERROR:  Some services needed are missing.  Run"
		eerror "        ""'./${SVCNAME}" $"broken' for a list of those"
		eerror "        " $"services."  "${SVCNAME}" $"was not started."
		retval=1	
	fi

	# Save the IN_BACKGROUND var as we need to clear it for starting depends
	local ib_save="${IN_BACKGROUND}"
	unset IN_BACKGROUND

	if [[ ${retval} == "0" && ${RC_NO_DEPS} != "yes" ]] ; then
		# Start dependencies, if any.
		local startsvc=$(rc-depend -ineed -iuse "${SVCNAME}")
		if ! is_runlevel_start ; then
			for x in ${startsvc} ; do
				service_stopped "${x}" && start_service "${x}"
			done
		fi

		# Wait for dependencies to finish.
		local ineed=$(rc-depend --notrace -ineed "${SVCNAME}")
		for x in ${startsvc} $(rc-depend -iafter "${SVCNAME}") ; do
			local timeout=3
			while [[ ${timeout} -gt 0 ]] ; do
				service_started "${x}" && continue 2
				wait_service "${x}"
				service_started "${x}" && continue 2
				if service_inactive "${x}" || service_scheduled "${x}" ; then
					if [[ " ${startsvc} " == *" ${x} "* ||
						" ${ineed} " == *" $(rc-depend --notrace -iprovide "${x}") " ]] ; then
						svc_schedule_start "${x}" "${SVCNAME}"
						[[ -n ${startinactive} ]] && startinactive="${startinactive}, "
						startinactive="${startinactive}${x}"
					fi

					continue 2
				fi
				service_stopped "${x}" && break
	
				((timeout--))
			done

			[[ " ${ineed} " != *" ${x} "*  ]] && continue
		
			eerror "ERROR:" $"cannot start" "${SVCNAME}" $"as" "${x}" $"could not start"
			retval=1
			break
		done

		if [[ -n ${startinactive} && ${retval} == "0" ]] ; then
			# Change the last , to or for correct grammar.
			x="${startinactive##*, }"
			startinactive="${startinactive/%, ${x}/ or ${x}}"
			ewarn "WARNING:" " ${SVCNAME}" $"is scheduled to start when" "${startinactive}" $"has started."
			retval=1
		fi
	fi
		
	if [[ ${retval} == "0" ]] ; then
		IN_BACKGROUND="${ib_save}"
		veindent
		(
		[[ ${RC_QUIET} == "yes" ]] && RC_QUIET_STDOUT="yes"
		exit() {
			eerror $"DO NOT USE EXIT IN INIT.D SCRIPTS"
			eerror $"This IS a bug, please fix your broken init.d"
			unset -f exit
			exit "$@"
		}

		# Apply any ulimits if defined
		[[ -n ${RC_ULIMIT} ]] && ulimit ${RC_ULIMIT}
		
		start
		<font color=red>)
		retval="$?"

		# Don't trust init scripts to reset indentation properly
		# Needed for ebuffer
		eoutdent 99999

		# If a service has been marked inactive, exit now as something
		# may attempt to start it again later
		if [[ ${retval} == "0" ]] &&</font> service_inactive "${SVCNAME}" ; then
			svcinactive=0
			ewarn $"WARNING:" " ${SVCNAME}" $"has started but is inactive"
			if [[ ${RC_PARALLEL_STARTUP} == "yes" && ${RC_QUIET} != "yes" ]] ; then
				eflush
				ebuffer ""
			fi
			return 1
		fi
	fi

	if [[ ${retval} != "0" ]] ; then
		if [[ ${svcinactive} == "0" ]] ; then
			mark_service_inactive "${SVCNAME}"
		elif [[ -z ${startinactive} ]] ; then
			mark_service_stopped "${SVCNAME}"
			is_runlevel_start && mark_service_failed "${SVCNAME}"
			eerror $"ERROR:" " ${SVCNAME}" $"failed to start"
		fi
	else
		svcstarted=0
		mark_service_started "${SVCNAME}"
		veinfo $"Service" "${SVCNAME}" $"started"
	fi

	# Flush the ebuffer
	if [[ ${RC_PARALLEL_STARTUP} == "yes" && ${RC_QUIET} != "yes" ]] ; then
		eflush
		ebuffer ""
	fi

	if ${svcbegin} ; then
		svcbegin=false
		end_service "${SVCNAME}"
	fi

	# Reset the trap
	svc_trap
	
	return "${retval}"
}
</pre>