FW: [RFC] A more general timeout specification

[Inaky Perez-Gonzalez <inaky@linux.intel.com>]

Hi John

Couple of months ago Joe Korty sent the attached proposal for a new
interface and I was wondering if you could comment on it.

The main user of this new inteface is to allow system calls to get
time specified in an absolute form (as most of POSIX states) and thus
avoid extra time conversion work.

There was a short thread about it, available at google groups
(grepping for the subject).

http://groups-beta.google.com/groups?q=a+more+general+timeout+specification

Thanks!

[ for comment only ]

The fusyn (robust mutexes) project proposes the creation
of a more general data structure, 'struct timeout', for the
specification of timeouts in new services.  In this structure,
the user specifies:

    a time, in timespec format.
    the clock the time is specified against (eg, CLOCK_MONOTONIC).
    whether the time is absolute, or relative to 'now'.

That is, all combinations of useful timeout attributes become
possible.

Also proposed are two new kernel routines for the manipulation
of timeouts:

	timeout_validate()
	timeout_sleep()

timeout_validate() error-checks the syntax of a timeout
argument and returns either zero or -EINVAL.  By breaking
timeout_validate() out from timeout_sleep(), it becomes possible
to error check the timeout 'far away' from the places in the
code where we would actually do the timeout, as well as being
able to perform such checks only at those places we know the
timeout specification is coming from an unsafe source.

timeout_sleep() puts the caller to sleep until the
specified end time is in the past, as measured against
the given clock, or until the caller is awakened by other
means (such as wake_up_process()).  Like schedule_timeout(),
TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE must be set ahead
of time; if TASK_INTERRUPTIBLE is set then signals will also
break the caller out of the sleep.

timeout_sleep() returns either 0 (returned early) or -ETIMEDOUT
(returned due to timeout).  It is up to the caller to resolve,
in the "returned early" case, why it returned early.

Timeout_sleep has a return argument, endtime, which is also in
'struct timeout' format.  If the input time was relative, then
it is converted to absolute and returned through this argument.
This can be used when an early-terminated service must be
restarted and side effects of the early termination-n-restart
(such as end time drift) are to be avoided.

Joe
"Money can buy bandwidth, but latency is forever" -- John Mashey


 2.6.12-rc4-jak/include/linux/time.h    |    6 +
 2.6.12-rc4-jak/include/linux/timeout.h |   48 ++++++++
 2.6.12-rc4-jak/kernel/posix-timers.c   |    7 +
 2.6.12-rc4-jak/kernel/timer.c          |  184 +++++++++++++++++++++++++++++++++
 4 files changed, 245 insertions(+)

diff -puNa include/linux/time.h~a.more.flexible.timeout.approach include/linux/time.h
--- 2.6.12-rc4/include/linux/time.h~a.more.flexible.timeout.approach	2005-05-18 13:53:14.204417169 -0400
+++ 2.6.12-rc4-jak/include/linux/time.h	2005-05-18 13:53:14.212416002 -0400
@@ -25,6 +25,8 @@ struct timezone {
 	int	tz_dsttime;	/* type of dst correction */
 };
 
+#include <linux/timeout.h>
+
 #ifdef __KERNEL__
 
 /* Parameters used to convert the timespec values */
@@ -103,6 +105,10 @@ struct itimerval;
 extern int do_setitimer(int which, struct itimerval *value, struct itimerval *ovalue);
 extern int do_getitimer(int which, struct itimerval *value);
 extern void getnstimeofday (struct timespec *tv);
+extern long clock_gettime(int which, struct timespec *tp);
+
+extern int FASTCALL(abs_timespec_to_abs_jiffies (clockid_t clock, const struct timespec *tp, unsigned long *jp));
+extern int FASTCALL(rel_to_abs_timespec(clockid_t clock, const struct timespec *tsrel, struct timespec *tsabs));
 
 extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
 
diff -puNa /dev/null include/linux/timeout.h
--- /dev/null	2004-06-24 14:04:38.000000000 -0400
+++ 2.6.12-rc4-jak/include/linux/timeout.h	2005-05-18 13:53:14.212416002 -0400
@@ -0,0 +1,48 @@
+/*
+ * Extended timeout specification
+ *
+ * (C) 2002-2005 Intel Corp
+ * Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>.
+ *
+ * Licensed under the FSF's GNU Public License v2 or later.
+ *
+ * Generic extended timeout specification.  Broken out by Joe Korty
+ * <joe.korty@ccur.com> from linux/time.h so that it can be included
+ * by userspace applications in conjunction with #include "time.h".
+ */
+
+#ifndef _LINUX_TIMEOUT_H
+#define _LINUX_TIMEOUT_H
+
+/* 'struct timeout' flag values.  OR these into clock_id along with
+ * a clock specification such as CLOCK_REALTIME or CLOCK_MONOTONIC.
+ */
+enum {
+	TIMEOUT_RELATIVE   = 0x10000000,	/* relative timeout */
+
+	TIMEOUT_FLAGS_MASK = 0xf0000000,	/* flags mask for clock_id */
+	TIMEOUT_CLOCK_MASK = 0x0fffffff,	/* clock mask for clock_id */
+};
+
+/* Magic values a 'struct timeout' pointer can have */
+
+#define TIMEOUT_MAX	((struct timeout *) ~0UL) /* never time out */
+#define TIMEOUT_NONE	((struct timeout *) 0UL)  /* time out immediately */
+
+/**
+ * struct timeout - general timeout specification
+ *
+ * @clock_id: which clock source to use ORed with flags describing use.
+ * @ts:       timespec for the timeout
+ */
+struct timeout {
+	clockid_t clock_id;
+	struct timespec ts;
+};
+
+#ifdef __KERNEL__
+extern int FASTCALL(timeout_validate (const struct timeout *));
+extern int FASTCALL(timeout_sleep (const struct timeout *, struct timeout *));
+#endif
+
+#endif
diff -puNa kernel/posix-timers.c~a.more.flexible.timeout.approach kernel/posix-timers.c
--- 2.6.12-rc4/kernel/posix-timers.c~a.more.flexible.timeout.approach	2005-05-18 13:53:14.207416731 -0400
+++ 2.6.12-rc4-jak/kernel/posix-timers.c	2005-05-18 13:53:14.214415710 -0400
@@ -1288,6 +1288,13 @@ sys_clock_settime(clockid_t which_clock,
 	return CLOCK_DISPATCH(which_clock, clock_set, (which_clock, &new_tp));
 }
 
+long clock_gettime(clockid_t which_clock, struct timespec *tp)
+{
+	if (invalid_clockid(which_clock))
+		return -EINVAL;
+	return CLOCK_DISPATCH(which_clock, clock_get, (which_clock, tp));
+}
+
 asmlinkage long
 sys_clock_gettime(clockid_t which_clock, struct timespec __user *tp)
 {
diff -puNa kernel/timer.c~a.more.flexible.timeout.approach kernel/timer.c
--- 2.6.12-rc4/kernel/timer.c~a.more.flexible.timeout.approach	2005-05-18 13:53:14.209416440 -0400
+++ 2.6.12-rc4-jak/kernel/timer.c	2005-05-18 15:27:06.363710594 -0400
@@ -1129,6 +1129,190 @@ fastcall signed long __sched schedule_ti
 
 EXPORT_SYMBOL(schedule_timeout);
 
+/**
+ * timeout_validate - verify that a timeout specification is self-consistant.
+ * @tp - pointer to the 'struct timeout' to verify
+ * @returns - 0 if no error, <0 errno code if there was
+ */
+fastcall int timeout_validate(const struct timeout *tp)
+{
+	int result;
+	unsigned flags, clock_id;
+
+	result = 0;
+	if (tp == TIMEOUT_MAX || tp == TIMEOUT_NONE)
+		goto out;
+
+	flags = tp->clock_id & TIMEOUT_FLAGS_MASK;
+	clock_id = tp->clock_id & TIMEOUT_CLOCK_MASK;
+
+	result = -EINVAL;
+	if (flags & ~TIMEOUT_RELATIVE)
+	    goto out;
+
+	/* someday, we should support *all* clocks available to us */
+	if (clock_id != CLOCK_REALTIME && clock_id != CLOCK_MONOTONIC)
+		goto out;
+	if ((unsigned long)tp->ts.tv_nsec >= NSEC_PER_SEC)
+		goto out;
+	result = 0;
+out:
+	return result;
+}
+EXPORT_SYMBOL_GPL(timeout_validate);
+
+/** rel_to_abs_timespec - convert a relative timespec into an absolute timespec
+ *  @clock - the system clock the timespecs are specified in.
+ *  @tsrel - a pointer to the relative timespec to be converted.
+ *  @tsabs - a pointer to where the absolute timespec is to be stored.
+ *  @returns - 0 if converted, <0 if error.
+ */
+fastcall int rel_to_abs_timespec(clockid_t clock, const struct timespec *tsrel,
+				struct timespec *tsabs)
+{
+	int result;
+	struct timespec now;
+
+	result = clock_gettime(clock, &now);
+	if (result >= 0) {
+		set_normalized_timespec(tsabs, now.tv_sec + tsrel->tv_sec,
+				now.tv_nsec + tsrel->tv_nsec);
+	}
+	return result;
+}
+EXPORT_SYMBOL_GPL(rel_to_abs_timespec);
+
+/**
+ * abs_timespec_to_abs_jiffies - convert an absolute timespec into
+ * absolute jiffies.
+ *
+ * @clock - select which clock @tp is specified against.  CLOCK_MONOTONIC and
+ *     CLOCK_REALTIME are two possibilities.
+ * @tp - absolute timespec that is to be converted to jiffies.
+ * @jp - absolute jiffies returned through this pointer.
+ * @returns -  0 if converted, <0 if error.
+ */
+fastcall int abs_timespec_to_abs_jiffies (clockid_t clock,
+			const struct timespec *tp, unsigned long *jp)
+{
+	int result;
+	unsigned long jiffies_abs, seq;
+	struct timespec oc, now;
+
+	do {
+		seq = read_seqbegin(&xtime_lock);
+		result = clock_gettime(clock, &now);
+		if (result < 0)
+			return result;
+		jiffies_abs = jiffies;
+	} while (read_seqretry(&xtime_lock, seq));
+
+	set_normalized_timespec(&oc, tp->tv_sec - now.tv_sec,
+				tp->tv_nsec - now.tv_nsec);
+
+	if (oc.tv_sec > 0 || (oc.tv_sec == 0 && oc.tv_nsec > 0))
+		jiffies_abs += timespec_to_jiffies(&oc) + 1;
+	*jp = jiffies_abs;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(abs_timespec_to_abs_jiffies);
+
+/* Helper function.  Handles the proper setting of @endtime when
+ * an @endtime needs to be returned.  Returns an approximation for
+ * the ending time in jiffies.  The approximation is guaranteed to
+ * be on or just past the true ending time.
+ */
+static inline
+unsigned long __timeout_sleep_timespec (const struct timeout *timeout,
+				struct timeout *endtime)
+{
+	unsigned long jiffies_abs;
+	struct timespec end;
+	clockid_t clock_id = timeout->clock_id & TIMEOUT_CLOCK_MASK;
+
+	if (timeout->clock_id & TIMEOUT_RELATIVE) {
+		rel_to_abs_timespec(clock_id, &timeout->ts, &end);
+		if (endtime) {
+			endtime->clock_id = clock_id;
+			endtime->ts = end;
+		}
+	}
+	else
+		end = timeout->ts;
+
+	abs_timespec_to_abs_jiffies(clock_id, &end, &jiffies_abs);
+	return jiffies_abs;
+}
+
+/**
+ * timeout_sleep - sleep until woken up, interrupted, or the
+ *		   designated wakeup time is past.
+ * @timeout:
+ *    time to wait.  const struct timeout pointer.  May take
+ *    on the special values TIMEOUT_MAX (never time out) or
+ *    TIMEOUT_NONE (times out instantly).  @timeout must be
+ *    error-checked by timeout_validate() beforehand.
+ * @endtime:
+ *    time remaining.  A 'struct timeout' value, holding the
+ *    end time of the period in absolute format and specified
+ *    against the same clock that @timeout was specified against,
+ *    is returned through this pointer.  No value is returned
+ *    if @endtime is NULL, if @timeout is TIMEOUT_MAX or
+ *    TIMEOUT_NONE, or if @timeout is itself specified in
+ *    absolute time.
+ * @returns:
+ *    -ETIMEDOUT if awakened due to timeout, 0 otherwise.
+ */
+fastcall int timeout_sleep (const struct timeout *timeout, struct timeout *endtime)
+{
+	unsigned long jiffies_endtime;
+	struct timer_list timer;
+	int result;
+
+	if (timeout == TIMEOUT_NONE)
+		goto simulate_timeout;
+
+	if (timeout == TIMEOUT_MAX)
+		goto never_timeout;
+
+	if (timeout->ts.tv_sec  == MAX_SCHEDULE_TIMEOUT)
+		goto never_timeout;
+
+	jiffies_endtime = __timeout_sleep_timespec(timeout, endtime);
+
+	if (time_after_eq(jiffies, jiffies_endtime))
+		goto simulate_timeout;
+
+	init_timer(&timer);
+	timer.expires = jiffies_endtime;
+	timer.data = (unsigned long)current;
+	timer.function = process_timeout;
+	add_timer(&timer);
+	schedule();
+	result = -ETIMEDOUT;
+	if (del_singleshot_timer_sync (&timer))
+		result = 0;
+	goto out;
+
+	/*
+	 * simulate a timeout without actually expiring a timer
+	 */
+simulate_timeout:
+	process_timeout((unsigned long)current);
+	result = -ETIMEDOUT;
+	goto out;
+
+	/*
+	 * Sleep without a timeout scheduled.
+	 */
+never_timeout:
+	schedule();
+	result = 0;
+out:
+	return result;
+}
+EXPORT_SYMBOL_GPL (timeout_sleep);
+
 /* Thread ID - the internal kernel "pid" */
 asmlinkage long sys_gettid(void)
 {

-- 

Inaky

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