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time-wait-sync: use watchfile to coordinate with timesyncd

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Systems that have an accurate real-time clock may have an initial
unsynchronized time that is close enough to the synchronized time that
the final adjustment doesn't trigger a waking "clock set" event.  Have
timesyncd touch a file in its runtime directory as a secondary signal
for synchronization.  Continue to support the timerfd-based trigger as a
sufficient condition when the watchfile is not present.

Closes issue #8683
This commit is contained in:
Peter A. Bigot 2018-04-09 13:39:16 -05:00
parent 006ffa6421
commit 2dd79846dd
5 changed files with 160 additions and 48 deletions
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30
man/systemd-time-wait-sync.service.xml
View file

@ -46,24 +46,30 @@
<title>Description</title>
<para><filename>systemd-time-wait-sync</filename> is a system service that delays the start of units that depend on
<filename>time-sync.target</filename> until <filename>systemd-timesyncd.service</filename> or something else has
set the system time and marked it as synchronized. Reaching this state generally requires synchronization with an
external source, such as an NTP server.</para>
<filename>time-sync.target</filename> until the system time has been synchronized with an accurate time source by
<filename>systemd-timesyncd.service</filename>.</para>
<para>When this unit is not enabled the <filename>time-sync.target</filename> synchronization point may be reached
as soon as the system time is advanced by <filename>systemd-timesyncd.service</filename> to the time stored at the
last shutdown. That time may not meet the expectations of dependent services that require an accurate
clock.</para>
<para><filename>systemd-timesyncd.service</filename> notifies on successful synchronization.
<filename>systemd-time-wait-sync</filename> also tries to detect when the kernel marks the time as synchronized,
but this detection is not reliable and is intended only as a fallback for other servies that can be used to
synchronize time (e.g., ntpd, chronyd).</para>
</refsect1>
<refsect1>
<title>Notes</title>
<title>Files</title>
<variablelist>
<varlistentry>
<term><filename>/run/systemd/timesync/synchronized</filename></term>
<listitem>
<para>The presence of this file indicates to this service that the system clock has been synchronized.</para>
</listitem>
</varlistentry>
</variablelist>
<para>This service works correctly with a time synchronization service like
<filename>systemd-timesyncd.service</filename> that uses the same protocol as NTP to set the time from a
synchronized source. When used with time synchronization services that follow a different protocol the event of
setting synchronized time may not be detected in which case this service will not complete.</para>
</refsect1>
<refsect1>

14
man/systemd-timesyncd.service.xml
View file

@ -80,10 +80,21 @@
<term><filename>/var/lib/systemd/timesync/clock</filename></term>
<listitem>
<para>This file contains the timestamp of the last successful
<para>The modification time of this file indicates the timestamp of the last successful
synchronization.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>/run/systemd/timesync/synchronized</filename></term>
<listitem>
<para>A file that is touched on each successful synchronization, to assist
<filename>systemd-time-wait-sync</filename> and other applications to detecting synchronization
events.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
@ -94,6 +105,7 @@
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>localtime</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>hwclock</refentrytitle><manvolnum>8</manvolnum></citerefentry>

162
src/time-wait-sync/time-wait-sync.c
View file

@ -25,6 +25,7 @@
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/inotify.h>
#include <sys/timerfd.h>
#include <sys/timex.h>
#include <unistd.h>
@ -32,32 +33,88 @@
#include "sd-event.h"
#include "fd-util.h"
#include "fs-util.h"
#include "missing.h"
#include "signal-util.h"
#include "time-util.h"
typedef struct ClockState {
int fd; /* non-negative is descriptor from timerfd_create */
int adjtime_state; /* return value from last adjtimex(2) call */
sd_event_source *event_source; /* non-null is the active io event source */
int timerfd_fd; /* non-negative is descriptor from timerfd_create */
int adjtime_state; /* return value from last adjtimex(2) call */
sd_event_source *timerfd_event_source; /* non-null is the active io event source */
int inotify_fd;
sd_event_source *inotify_event_source;
int run_systemd_wd;
int run_systemd_timesync_wd;
bool has_watchfile;
} ClockState;
static void clock_state_release_timerfd(ClockState *sp) {
sp->timerfd_event_source = sd_event_source_unref(sp->timerfd_event_source);
sp->timerfd_fd = safe_close(sp->timerfd_fd);
}
static void clock_state_release(ClockState *sp) {
sp->event_source = sd_event_source_unref(sp->event_source);
sp->fd = safe_close(sp->fd);
clock_state_release_timerfd(sp);
sp->inotify_event_source = sd_event_source_unref(sp->inotify_event_source);
sp->inotify_fd = safe_close(sp->inotify_fd);
}
static int clock_state_update(ClockState *sp, sd_event *event);
static int io_handler(sd_event_source * s,
int fd,
uint32_t revents,
void *userdata) {
static int update_notify_run_systemd_timesync(ClockState *sp) {
sp->run_systemd_timesync_wd = inotify_add_watch(sp->inotify_fd, "/run/systemd/timesync", IN_CREATE|IN_DELETE_SELF);
return sp->run_systemd_timesync_wd;
}
static int timerfd_handler(sd_event_source *s,
int fd,
uint32_t revents,
void *userdata) {
ClockState *sp = userdata;
return clock_state_update(sp, sd_event_source_get_event(s));
}
static void process_inotify_event(sd_event *event, ClockState *sp, struct inotify_event *e) {
if (e->wd == sp->run_systemd_wd) {
/* Only thing we care about is seeing if we can start watching /run/systemd/timesync. */
if (sp->run_systemd_timesync_wd < 0)
update_notify_run_systemd_timesync(sp);
} else if (e->wd == sp->run_systemd_timesync_wd) {
if (e->mask & IN_DELETE_SELF) {
/* Somebody removed /run/systemd/timesync. */
(void) inotify_rm_watch(sp->inotify_fd, sp->run_systemd_timesync_wd);
sp->run_systemd_timesync_wd = -1;
} else
/* Somebody might have created /run/systemd/timesync/synchronized. */
clock_state_update(sp, event);
}
}
static int inotify_handler(sd_event_source *s,
int fd,
uint32_t revents,
void *userdata) {
sd_event *event = sd_event_source_get_event(s);
ClockState *sp = userdata;
union inotify_event_buffer buffer;
struct inotify_event *e;
ssize_t l;
l = read(fd, &buffer, sizeof(buffer));
if (l < 0) {
if (IN_SET(errno, EAGAIN, EINTR))
return 0;
return log_warning_errno(errno, "Lost access to inotify: %m");
}
FOREACH_INOTIFY_EVENT(e, buffer, l)
process_inotify_event(event, sp, e);
return 0;
}
static int clock_state_update(ClockState *sp,
sd_event *event) {
static const struct itimerspec its = {
@ -69,34 +126,39 @@ static int clock_state_update(ClockState *sp,
usec_t t;
const char * ts;
clock_state_release(sp);
clock_state_release_timerfd(sp);
/* The kernel supports cancelling timers whenever its realtime clock is "set" (which can happen in a variety of
* ways, generally adjustments of at least 500 ms). The way this module works is we set up a timer that will
* ways, generally adjustments of at least 500 ms). The way this module works is we set up a timerfd that will
* wake when the clock is set, and when that happens we read the clock synchronization state from the return
* value of adjtimex(2), which supports the NTP time adjustment protocol.
*
* The kernel determines whether the clock is synchronized using driver-specific tests, based on time
* information passed by an application, generally through adjtimex(2). If the application asserts the clock
* is synchronized, but does not also do something that "sets the clock", the timer will not be cancelled and
* synchronization will not be detected. Should this behavior be observed with a time synchronization provider
* this code might be reworked to do a periodic check as well.
* information passed by an application, generally through adjtimex(2). If the application asserts the clock is
* synchronized, but does not also do something that "sets the clock", the timer will not be cancelled and
* synchronization will not be detected.
*
* Similarly, this service will never complete if the application sets the time without also providing
* information that adjtimex(2) can use to determine that the clock is synchronized.
* information that adjtimex(2) can use to determine that the clock is synchronized. This generally doesn't
* happen, but can if the system has a hardware clock that is accurate enough that the adjustment is too small
* to be a "set".
*
* Well-behaved implementations including systemd-timesyncd should not produce either situation. For timesyncd
* the initial setting of the time uses settimeofday(2), which sets the clock but does not mark it
* synchronized. When an NTP source is selected it sets the clock again with clock_adjtime(2) which does mark
* it synchronized. */
* Both these failure-to-detect situations are covered by having the presence/creation of
* /run/systemd/timesync/synchronized, which is considered sufficient to indicate a synchronized clock even if
* the kernel has not been updated.
*
* For timesyncd the initial setting of the time uses settimeofday(2), which sets the clock but does not mark
* it synchronized. When an NTP source is selected it sets the clock again with clock_adjtime(2) which marks it
* synchronized and also touches /run/systemd/timesync/synchronized which covers the case when the clock wasn't
* "set". */
r = timerfd_create(CLOCK_REALTIME, TFD_NONBLOCK | TFD_CLOEXEC);
if (r < 0) {
log_error_errno(errno, "Failed to create timerfd: %m");
goto finish;
}
sp->fd = r;
sp->timerfd_fd = r;
r = timerfd_settime(sp->fd, TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET, &its, NULL);
r = timerfd_settime(sp->timerfd_fd, TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET, &its, NULL);
if (r < 0) {
log_error_errno(errno, "Failed to set timerfd conditions: %m");
goto finish;
@ -117,24 +179,26 @@ static int clock_state_update(ClockState *sp,
strcpy(buf, "unrepresentable");
log_info("adjtime state %d status %x time %s", sp->adjtime_state, tx.status, ts);
if (sp->adjtime_state == TIME_ERROR) {
/* Not synchronized. Do a one-shot wait on the descriptor and inform the caller we need to keep
sp->has_watchfile = access("/run/systemd/timesync/synchronized", F_OK) >= 0;
if (sp->has_watchfile)
/* Presence of watch file overrides adjtime_state */
r = 0;
else if (sp->adjtime_state == TIME_ERROR) {
/* Not synchronized. Do a one-shot wait on the descriptor and inform the caller we need to keep
* running. */
r = sd_event_add_io(event, &sp->event_source, sp->fd,
EPOLLIN, io_handler, sp);
r = sd_event_add_io(event, &sp->timerfd_event_source, sp->timerfd_fd,
EPOLLIN, timerfd_handler, sp);
if (r < 0) {
log_error_errno(r, "Failed to create time change monitor source: %m");
goto finish;
}
r = 1;
} else {
} else
/* Synchronized; we can exit. */
(void) sd_event_exit(event, 0);
r = 0;
}
finish:
if (r < 0)
if (r <= 0)
(void) sd_event_exit(event, r);
return r;
}
@ -143,7 +207,10 @@ int main(int argc, char * argv[]) {
int r;
_cleanup_(sd_event_unrefp) sd_event *event;
ClockState state = {
.fd = -1,
.timerfd_fd = -1,
.inotify_fd = -1,
.run_systemd_wd = -1,
.run_systemd_timesync_wd = -1,
};
assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT, -1) >= 0);
@ -172,17 +239,42 @@ int main(int argc, char * argv[]) {
goto finish;
}
r = inotify_init1(IN_NONBLOCK|IN_CLOEXEC);
if (r < 0) {
log_error_errno(errno, "Failed to create inotify descriptor: %m");
goto finish;
}
state.inotify_fd = r;
r = sd_event_add_io(event, &state.inotify_event_source, state.inotify_fd,
EPOLLIN, inotify_handler, &state);
if (r < 0) {
log_error_errno(r, "Failed to create notify event source: %m");
goto finish;
}
r = inotify_add_watch(state.inotify_fd, "/run/systemd/", IN_CREATE);
if (r < 0) {
log_error_errno(errno, "Failed to watch /run/systemd/: %m");
goto finish;
}
state.run_systemd_wd = r;
(void) update_notify_run_systemd_timesync(&state);
r = clock_state_update(&state, event);
if (r > 0) {
r = sd_event_loop(event);
if (r < 0)
log_error_errno(r, "Failed in event loop: %m");
else if (state.adjtime_state == TIME_ERROR) {
log_error("Event loop terminated without synchronizing");
r = -ECANCELED;
}
}
if (state.has_watchfile)
log_debug("Exit enabled by: /run/systemd/timesync/synchonized");
if (state.adjtime_state == TIME_ERROR)
log_info("Exit without adjtimex synchronized.");
finish:
clock_state_release(&state);
return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;

1
src/timesync/timesyncd-manager.c
View file

@ -355,6 +355,7 @@ static int manager_adjust_clock(Manager *m, double offset, int leap_sec) {
/* If touch fails, there isn't much we can do. Maybe it'll work next time. */
(void) touch("/var/lib/systemd/timesync/clock");
(void) touch("/run/systemd/timesync/synchronized");
m->drift_ppm = tmx.freq / 65536;

1
units/systemd-timesyncd.service.in
View file

@ -37,6 +37,7 @@ MemoryDenyWriteExecute=yes
RestrictRealtime=yes
RestrictNamespaces=yes
RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
RuntimeDirectory=systemd/timesync
SystemCallFilter=~@cpu-emulation @debug @keyring @module @mount @obsolete @raw-io @reboot @swap
SystemCallArchitectures=native
LockPersonality=yes