Re: [patch 3/3] timerfd: Implement write method

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thu, May 22, 2014 at 06:58:19AM +0900, Thomas Gleixner wrote:
> > 
> > So what wakes a potential waiter in read/poll?
> 
> And who is updating timerfd_create(2) ?

Thomas, could you please take a look if the approach below is acceptable?
If it will be fine I update manpage then.
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: timerfd: Implement timerfd_ioctl method to restore timerfd_ctx::ticks

The read() of timerfd files allows to fetch the number of timer ticks
while there is no way to set it back from userspace.

To restore the timer's state as it was at checkpoint moment we need
a path to bring @ticks back. Initially I thought about writing ticks
back via write() interface but it seems such API is somehow obscure.

Instead implement timerfd_ioctl() method with TFD_IOC_SET_TICKS
command which requires CAP_SYS_RESOURCE capability to be able to
set @ticks into arbitrary value. Note this command doesn't wake
up readers/waiters and its purpose only to serve C/R needs
(for same sake I wrapped code with CONFIG_CHECKPOINT_RESTORE).
Still if needed the ioctl may be extended for new commands
and CONFIG_CHECKPOINT_RESTORE dropped off.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Andrey Vagin <avagin@openvz.org>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
 fs/timerfd.c            |   31 +++++++++++++++++++++++++++++++
 include/linux/timerfd.h |    5 +++++
 2 files changed, 36 insertions(+)

Index: linux-2.6.git/fs/timerfd.c
===================================================================
--- linux-2.6.git.orig/fs/timerfd.c
+++ linux-2.6.git/fs/timerfd.c
@@ -313,11 +313,42 @@ static int timerfd_show(struct seq_file
 }
 #endif
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+static long timerfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct timerfd_ctx *ctx = file->private_data;
+	int ret = 0;
+
+	switch (cmd) {
+	case TFD_IOC_SET_TICKS: {
+		u64 ticks;
+
+		if (!capable(CAP_SYS_RESOURCE))
+			return -EPERM;
+		if (get_user(ticks, (u64 __user *)arg))
+			return -EFAULT;
+		spin_lock_irq(&ctx->wqh.lock);
+		ctx->ticks = ticks;
+		spin_unlock_irq(&ctx->wqh.lock);
+		break;
+	}
+	default:
+		ret = -ENOTTY;
+		break;
+	}
+
+	return ret;
+}
+#endif
+
 static const struct file_operations timerfd_fops = {
 	.release	= timerfd_release,
 	.poll		= timerfd_poll,
 	.read		= timerfd_read,
 	.llseek		= noop_llseek,
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	.unlocked_ioctl	= timerfd_ioctl,
+#endif
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= timerfd_show,
 #endif
Index: linux-2.6.git/include/linux/timerfd.h
===================================================================
--- linux-2.6.git.orig/include/linux/timerfd.h
+++ linux-2.6.git/include/linux/timerfd.h
@@ -11,6 +11,9 @@
 /* For O_CLOEXEC and O_NONBLOCK */
 #include <linux/fcntl.h>
 
+/* For _IO helpers */
+#include <linux/ioctl.h>
+
 /*
  * CAREFUL: Check include/asm-generic/fcntl.h when defining
  * new flags, since they might collide with O_* ones. We want
@@ -29,4 +32,6 @@
 /* Flags for timerfd_settime.  */
 #define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET)
 
+#define TFD_IOC_SET_TICKS	_IOW('T', 0, u64)
+
 #endif /* _LINUX_TIMERFD_H */