Re: Why does timerfd() only support CLOCK_REALTIME and CLOCK_MONOTONIC?

Davide Libenzi
2009-02-08T20:47:14Z

On Sun, 8 Feb 2009, Davide Libenzi wrote:

> On Mon, 9 Feb 2009, Michael Kerrisk wrote:
>
> > Hi Davide,
> >
> > At the moment I'm looking into writing man pages for timer_create(2)
> > and friends. (Somewhat bizarrely, these pages do not yet exist.) As
> > I looked into the source code of timer_create(), etc., and did some
> > tests, I saw that timer_create() supports the following clocks:
> >
> > TIMER_REALTIME
> > TIMER_MONOTONIC
> > TIMER_PROCESS_CPUTIME_ID
> > TIMER_THREAD_CPUTIME_ID
> > clockid obtained from clock_getcpuclockid(3)
> > clockid obtained from pthread_getcpuclockid(3)
> >
> > On the other hand, timerfd() only permits the first two of these.
> > What's the reason for that limitation of timerfd()? (It may be worth
> > adding something to the man page on this point.)
>
> No particular reason I can think of. If Thomas makes invalid_clockid()
> available, we could allow timerfd() to support all time timers
> timer_create() supports.
> Do you see any reason why this won't work Thomas?

Patch, not tested, below.

- Davide

---
fs/timerfd.c | 10 ++++------
include/linux/posix-timers.h | 1 +
include/linux/timerfd.h | 13 ++++++++++---
kernel/posix-timers.c | 2 +-
4 files changed, 16 insertions(+), 10 deletions(-)

Index: linux-2.6.mod/fs/timerfd.c
===================================================================
--- linux-2.6.mod.orig/fs/timerfd.c 2009-02-08 12:32:25.000000000 -0800
+++ linux-2.6.mod/fs/timerfd.c 2009-02-08 12:43:57.000000000 -0800
@@ -18,6 +18,7 @@
#include <linux/spinlock.h>
#include <linux/time.h>
#include <linux/hrtimer.h>
+#include <linux/posix-timers.h>
#include <linux/anon_inodes.h>
#include <linux/timerfd.h>
#include <linux/syscalls.h>
@@ -186,12 +187,9 @@ SYSCALL_DEFINE2(timerfd_create, int, clo
BUILD_BUG_ON(TFD_CLOEXEC != O_CLOEXEC);
BUILD_BUG_ON(TFD_NONBLOCK != O_NONBLOCK);

- if (flags & ~(TFD_CLOEXEC | TFD_NONBLOCK))
+ if ((flags & ~TFD_FLAGS_SET) ||
+ invalid_clockid(clockid))
return -EINVAL;
- if (clockid != CLOCK_MONOTONIC &&
- clockid != CLOCK_REALTIME)
- return -EINVAL;
-
ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
if (!ctx)
return -ENOMEM;
@@ -201,7 +199,7 @@ SYSCALL_DEFINE2(timerfd_create, int, clo
hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);

ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
- flags & (O_CLOEXEC | O_NONBLOCK));
+ flags & TFD_SHARED_FCNTL_FLAGS);
if (ufd < 0)
kfree(ctx);

Index: linux-2.6.mod/include/linux/posix-timers.h
===================================================================
--- linux-2.6.mod.orig/include/linux/posix-timers.h 2009-02-08 12:32:25.000000000 -0800
+++ linux-2.6.mod/include/linux/posix-timers.h 2009-02-08 12:33:12.000000000 -0800
@@ -84,6 +84,7 @@ struct k_clock {
struct itimerspec * cur_setting);
};

+int invalid_clockid(const clockid_t which_clock);
void register_posix_clock(const clockid_t clock_id, struct k_clock *new_clock);

/* error handlers for timer_create, nanosleep and settime */
Index: linux-2.6.mod/kernel/posix-timers.c
===================================================================
--- linux-2.6.mod.orig/kernel/posix-timers.c 2009-02-08 12:32:25.000000000 -0800
+++ linux-2.6.mod/kernel/posix-timers.c 2009-02-08 12:32:57.000000000 -0800
@@ -205,7 +205,7 @@ static int no_timer_create(struct k_itim
/*
* Return nonzero if we know a priori this clockid_t value is bogus.
*/
-static inline int invalid_clockid(const clockid_t which_clock)
+int invalid_clockid(const clockid_t which_clock)
{
if (which_clock < 0) /* CPU clock, posix_cpu_* will check it */
return 0;
Index: linux-2.6.mod/include/linux/timerfd.h
===================================================================
--- linux-2.6.mod.orig/include/linux/timerfd.h 2009-02-08 12:34:25.000000000 -0800
+++ linux-2.6.mod/include/linux/timerfd.h 2009-02-08 12:36:09.000000000 -0800
@@ -11,13 +11,20 @@
/* For O_CLOEXEC and O_NONBLOCK */
#include <linux/fcntl.h>

-/* Flags for timerfd_settime. */
+/*
+ * CAREFUL: Check include/asm-generic/fcntl.h when defining
+ * new flags, since they might collide with O_* ones. We want
+ * to re-use O_* flags that couldn't possibly have a meaning
+ * from eventfd, in order to leave a free define-space for
+ * shared O_* flags.
+ */
#define TFD_TIMER_ABSTIME (1 << 0)
-
-/* Flags for timerfd_create. */
#define TFD_CLOEXEC O_CLOEXEC
#define TFD_NONBLOCK O_NONBLOCK

+#define TFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
+#define TFD_FLAGS_SET (TFD_SHARED_FCNTL_FLAGS | TFD_TIMER_ABSTIME)
+

#endif /* _LINUX_TIMERFD_H */