On Wed, Mar 09, 2011 at 04:36:51PM +0200, Alexander Shishkin wrote: > 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(¬ifiers_lock); > + list_for_each_entry(ctx, ¬ifiers_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(¬ifiers_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, ¬ifiers_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(¬ifiers_lock); > + list_del(&ctx->notifiers_list); > + spin_unlock(¬ifiers_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; Whitespace changes? > @@ -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(¬ifiers_lock); > spin_lock_irq(&ctx->wqh.lock); > - if (hrtimer_try_to_cancel(&ctx->tmr) >= 0) > + if (!list_empty(¬ifiers_list) || hrtimer_try_to_cancel(&ctx->tmr) >= 0) > break; > spin_unlock_irq(&ctx->wqh.lock); > + spin_unlock(¬ifiers_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(¬ifiers_lock); > fput(file); > if (otmr && copy_to_user(otmr, &kotmr, sizeof(kotmr))) > return -EFAULT; What's reason to do copy_to_user() in case of error? Is it safe for error from security_settime()? > - 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; > } This lable is not needed. > 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); > + > } > > /* Whitespace changes? > @@ -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 > -- Kirill A. 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