[RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

From: Alexander Shishkin
Date: Wed Mar 09 2011 - 09:37:59 EST

Changes since v3:
- changed timerfd_settime() semantics (see below)
Changes since v2:
- replaced sysfs interface with a syscall
- added sysctl/procfs handle to set a limit to the number of users
- fixed issues pointed out by Greg.
Changes since v1:
- updated against 2.6.36-rc1,
- added notification/filtering options,
- added Documentation/ABI/sysfs-kernel-time-notify interface description.

Certain userspace applications (like "clock" desktop applets or cron or
systemd) might want to be notified when some other application changes
the system time. There are several known to me reasons for this:
- avoiding periodic wakeups to poll time changes;
- rearming CLOCK_REALTIME timers when said changes happen;
- changing system timekeeping policy for system-wide time management
programs;
- keeping guest applications/operating systems running in emulators
up to date;
- recalculation of traffic signal cycles in Advanced Traffic Controllers
(ATC), which is part of ATC API requirements [1] as developed by ATC
working group of the U.S. Institute of Transportation Engineers (ITE).

The major change since the previous version is the new semantics of
timerfd_settime() when it's called on a time change notification
descriptor: it will set the system time to utmr.it_value if the time
change counter is zero, otherwise it will return EBUSY, this is required
to prevent a race between setting the time and reading the counter, when
the time controlling procees changes the time immediately after another
process in the system did the same (the counter is greater than one),
that process' time change will be lost. Thus, the time controlling
process should use timerfd_settime() instead of clock_settime() or
settimeofday() to ensure that other processes' time changes don't get
lost.

This is another attempt to approach notifying userspace about system
clock changes. The other one is using an eventfd and a syscall [1]. In
the course of discussing the necessity of a syscall for this kind of
notifications, it was suggested that this functionality can be achieved
via timers [2] (and timerfd in particular [3]). This idea got quite
some support [4], [5], [6] and some vague criticism [7], so I decided
to try and go a bit further with it.

[1] http://www.ite.org/standards/atcapi/version2.asp
[2] http://marc.info/?l=linux-kernel&m=128950389423614&w=2
[3] http://marc.info/?l=linux-kernel&m=128951020831573&w=2
[4] http://marc.info/?l=linux-kernel&m=128951588006157&w=2
[5] http://marc.info/?l=linux-kernel&m=128951503205111&w=2
[6] http://marc.info/?l=linux-kernel&m=128955890118477&w=2
[7] http://marc.info/?l=linux-kernel&m=129002967031104&w=2
[8] http://marc.info/?l=linux-kernel&m=129002672227263&w=2

Signed-off-by: Alexander Shishkin <virtuoso@xxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxx>
CC: Feng Tang <feng.tang@xxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Michael Tokarev <mjt@xxxxxxxxxx>
CC: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
CC: John Stultz <johnstul@xxxxxxxxxx>
CC: Chris Friesen <chris.friesen@xxxxxxxxxxx>
CC: Kay Sievers <kay.sievers@xxxxxxxx>
CC: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
CC: Artem Bityutskiy <dedekind1@xxxxxxxxx>
CC: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
CC: linux-fsdevel@xxxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
---
fs/timerfd.c | 94 ++++++++++++++++++++++++++++++++++++++++++-----
include/linux/hrtimer.h | 6 +++
include/linux/timerfd.h | 3 +-
kernel/compat.c | 5 ++-
kernel/hrtimer.c | 4 ++
kernel/time.c | 11 ++++-
6 files changed, 109 insertions(+), 14 deletions(-)

diff --git a/fs/timerfd.c b/fs/timerfd.c
index 8c4fc14..6170f61 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -22,6 +22,7 @@
#include <linux/anon_inodes.h>
#include <linux/timerfd.h>
#include <linux/syscalls.h>
+#include <linux/security.h>

struct timerfd_ctx {
struct hrtimer tmr;
@@ -30,8 +31,13 @@ struct timerfd_ctx {
u64 ticks;
int expired;
int clockid;
+ struct list_head notifiers_list;
};

+/* TFD_NOTIFY_CLOCK_SET timers go here */
+static DEFINE_SPINLOCK(notifiers_lock);
+static LIST_HEAD(notifiers_list);
+
/*
* This gets called when the timer event triggers. We set the "expired"
* flag, but we do not re-arm the timer (in case it's necessary,
@@ -51,10 +57,31 @@ static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
return HRTIMER_NORESTART;
}

+void timerfd_clock_was_set(clockid_t clockid)
+{
+ struct timerfd_ctx *ctx;
+ unsigned long flags;
+
+ spin_lock(&notifiers_lock);
+ list_for_each_entry(ctx, &notifiers_list, notifiers_list) {
+ spin_lock_irqsave(&ctx->wqh.lock, flags);
+ if (ctx->tmr.base->index == clockid) {
+ ctx->ticks++;
+ wake_up_locked(&ctx->wqh);
+ }
+ spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+ }
+ spin_unlock(&notifiers_lock);
+}
+
static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
{
ktime_t remaining;

+ /* for notification timers, return current time */
+ if (!list_empty(&ctx->notifiers_list))
+ return timespec_to_ktime(current_kernel_time());
+
remaining = hrtimer_expires_remaining(&ctx->tmr);
return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}
@@ -72,6 +99,12 @@ static void timerfd_setup(struct timerfd_ctx *ctx, int flags,
ctx->expired = 0;
ctx->ticks = 0;
ctx->tintv = timespec_to_ktime(ktmr->it_interval);
+
+ if (flags & TFD_NOTIFY_CLOCK_SET) {
+ list_add(&ctx->notifiers_list, &notifiers_list);
+ return;
+ }
+
hrtimer_init(&ctx->tmr, ctx->clockid, htmode);
hrtimer_set_expires(&ctx->tmr, texp);
ctx->tmr.function = timerfd_tmrproc;
@@ -83,7 +116,12 @@ static int timerfd_release(struct inode *inode, struct file *file)
{
struct timerfd_ctx *ctx = file->private_data;

- hrtimer_cancel(&ctx->tmr);
+ if (!list_empty(&ctx->notifiers_list)) {
+ spin_lock(&notifiers_lock);
+ list_del(&ctx->notifiers_list);
+ spin_unlock(&notifiers_lock);
+ } else
+ hrtimer_cancel(&ctx->tmr);
kfree(ctx);
return 0;
}
@@ -113,6 +151,7 @@ static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,

if (count < sizeof(ticks))
return -EINVAL;
+
spin_lock_irq(&ctx->wqh.lock);
if (file->f_flags & O_NONBLOCK)
res = -EAGAIN;
@@ -120,7 +159,8 @@ static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
res = wait_event_interruptible_locked_irq(ctx->wqh, ctx->ticks);
if (ctx->ticks) {
ticks = ctx->ticks;
- if (ctx->expired && ctx->tintv.tv64) {
+ if (ctx->expired && ctx->tintv.tv64 &&
+ list_empty(&ctx->notifiers_list)) {
/*
* If tintv.tv64 != 0, this is a periodic timer that
* needs to be re-armed. We avoid doing it in the timer
@@ -184,6 +224,8 @@ SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
ctx->clockid = clockid;
hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);

+ INIT_LIST_HEAD(&ctx->notifiers_list);
+
ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));
if (ufd < 0)
@@ -196,18 +238,24 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
const struct itimerspec __user *, utmr,
struct itimerspec __user *, otmr)
{
+ int ret = 0;
struct file *file;
struct timerfd_ctx *ctx;
struct itimerspec ktmr, kotmr;

- if (copy_from_user(&ktmr, utmr, sizeof(ktmr)))
- return -EFAULT;
-
- if ((flags & ~TFD_SETTIME_FLAGS) ||
- !timespec_valid(&ktmr.it_value) ||
- !timespec_valid(&ktmr.it_interval))
+ if (flags & ~TFD_SETTIME_FLAGS)
return -EINVAL;

+ /* utmr may be NULL for notification timerfd */
+ if (!(flags & TFD_NOTIFY_CLOCK_SET) || utmr) {
+ if (copy_from_user(&ktmr, utmr, sizeof(ktmr)))
+ return -EFAULT;
+
+ if (!timespec_valid(&ktmr.it_value) ||
+ !timespec_valid(&ktmr.it_interval))
+ return -EINVAL;
+ }
+
file = timerfd_fget(ufd);
if (IS_ERR(file))
return PTR_ERR(file);
@@ -218,10 +266,12 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
* it to the new values.
*/
for (;;) {
+ spin_lock(&notifiers_lock);
spin_lock_irq(&ctx->wqh.lock);
- if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
+ if (!list_empty(&notifiers_list) || hrtimer_try_to_cancel(&ctx->tmr) >= 0)
break;
spin_unlock_irq(&ctx->wqh.lock);
+ spin_unlock(&notifiers_lock);
cpu_relax();
}

@@ -238,16 +288,39 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
kotmr.it_interval = ktime_to_timespec(ctx->tintv);

/*
+ * for the notification timerfd, set current time to it_value
+ * if the timer hasn't expired; otherwise someone has changed
+ * the system time to the value that we don't know
+ */
+ if (!list_empty(&ctx->notifiers_list) && utmr) {
+ if (ctx->ticks) {
+ ret = -EBUSY;
+ goto out;
+ }
+
+ ret = security_settime(&ktmr.it_value, NULL);
+ if (ret)
+ goto out;
+
+ spin_unlock_irq(&ctx->wqh.lock);
+ ret = do_settimeofday(&ktmr.it_value);
+ goto out1;
+ }
+
+ /*
* Re-program the timer to the new value ...
*/
timerfd_setup(ctx, flags, &ktmr);

+out:
spin_unlock_irq(&ctx->wqh.lock);
+out1:
+ spin_unlock(&notifiers_lock);
fput(file);
if (otmr && copy_to_user(otmr, &kotmr, sizeof(kotmr)))
return -EFAULT;

- return 0;
+ return ret;
}

SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
@@ -273,6 +346,7 @@ SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
spin_unlock_irq(&ctx->wqh.lock);
fput(file);

+out:
return copy_to_user(otmr, &kotmr, sizeof(kotmr)) ? -EFAULT: 0;
}

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 6bc1804..991e8d9 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -248,6 +248,12 @@ static inline ktime_t hrtimer_expires_remaining(const struct hrtimer *timer)
return ktime_sub(timer->node.expires, timer->base->get_time());
}

+#ifdef CONFIG_TIMERFD
+extern void timerfd_clock_was_set(clockid_t clockid);
+#else
+static inline void timerfd_clock_was_set(clockid_t clockid) {}
+#endif
+
#ifdef CONFIG_HIGH_RES_TIMERS
struct clock_event_device;

diff --git a/include/linux/timerfd.h b/include/linux/timerfd.h
index 2d07929..c3ddad9 100644
--- a/include/linux/timerfd.h
+++ b/include/linux/timerfd.h
@@ -19,6 +19,7 @@
* shared O_* flags.
*/
#define TFD_TIMER_ABSTIME (1 << 0)
+#define TFD_NOTIFY_CLOCK_SET (1 << 1)
#define TFD_CLOEXEC O_CLOEXEC
#define TFD_NONBLOCK O_NONBLOCK

@@ -26,6 +27,6 @@
/* Flags for timerfd_create. */
#define TFD_CREATE_FLAGS TFD_SHARED_FCNTL_FLAGS
/* Flags for timerfd_settime. */
-#define TFD_SETTIME_FLAGS TFD_TIMER_ABSTIME
+#define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_NOTIFY_CLOCK_SET)

#endif /* _LINUX_TIMERFD_H */
diff --git a/kernel/compat.c b/kernel/compat.c
index 38b1d2c..b1cf3e1 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -995,7 +995,10 @@ asmlinkage long compat_sys_stime(compat_time_t __user *tptr)
if (err)
return err;

- do_settimeofday(&tv);
+ err = do_settimeofday(&tv);
+ if (!err)
+ timerfd_clock_was_set(CLOCK_REALTIME);
+
return 0;
}

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 2c3d6e5..469eef6 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -663,6 +663,7 @@ void clock_was_set(void)
{
/* Retrigger the CPU local events everywhere */
on_each_cpu(retrigger_next_event, NULL, 1);
+
}

/*
@@ -675,6 +676,9 @@ void hres_timers_resume(void)
KERN_INFO "hres_timers_resume() called with IRQs enabled!");

retrigger_next_event(NULL);
+
+ /* Trigger timerfd notifiers */
+ timerfd_clock_was_set(CLOCK_MONOTONIC);
}

/*
diff --git a/kernel/time.c b/kernel/time.c
index 6430a75..b06f759 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -92,7 +92,10 @@ SYSCALL_DEFINE1(stime, time_t __user *, tptr)
if (err)
return err;

- do_settimeofday(&tv);
+ err = do_settimeofday(&tv);
+ if (!err)
+ timerfd_clock_was_set(CLOCK_REALTIME);
+
return 0;
}

@@ -177,7 +180,11 @@ int do_sys_settimeofday(const struct timespec *tv, const struct timezone *tz)
/* SMP safe, again the code in arch/foo/time.c should
* globally block out interrupts when it runs.
*/
- return do_settimeofday(tv);
+ error = do_settimeofday(tv);
+ if (!error)
+ timerfd_clock_was_set(CLOCK_REALTIME);
+
+ return error;
}
return 0;
}
--
1.7.2.1.45.gb66c2

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