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

Michael Kerrisk
2009-02-08T21:18:34Z

On Mon, Feb 9, 2009 at 9:46 AM, Davide Libenzi <davidel@xmailserver.org> wrote:
> 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.

(Has it been compiled?)

A further comment below.

> ---
> 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)

s/O_/TFD_/g above

> +#define TFD_FLAGS_SET (TFD_SHARED_FCNTL_FLAGS | TFD_TIMER_ABSTIME)
> +
>
> #endif /* _LINUX_TIMERFD_H */

Cheers,

Michael