Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable
From: John Stultz
Date: Mon Nov 19 2012 - 13:57:17 EST
On 11/19/2012 09:45 AM, Kees Cook wrote:
On Mon, Nov 19, 2012 at 9:23 AM, John Stultz <johnstul@xxxxxxxxxx> wrote:
On 11/18/2012 12:09 PM, Kees Cook wrote:
On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@xxxxxxxxxx> wrote:
Yea, I wanted to revisit this, because it is an odd case.
We don't want to call getnstimeofday() while the timekeeping code is
suspended, since the clocksource cycle_last value may be invalid if the
hardware was reset during suspend. Kees is correct, the WARN_ONs were
there to make sure no one tries to use the timekeeping core before its
resumed, so removing them is problematic.
Your sugggestion of having the __do_gettimeofday() internal accessor that
maybe returns an error if timekeeping has been suspended could work.
The other possibility is depending on the needs for accuracy with the
timestamp, current_kernel_time() might be a better interface to use,
since
it will return the time at the last tick, and doesn't require accessing
the
clocksource hardware. Might that be a simpler solution? Or is sub-tick
granularity necessary?
I think it's only useful to have this to the same granularity as
sched_clock(), so things can be correlated to dmesg output. If it's
the same, I'd be fine to switch to using current_kernel_time().
Oof. That's another can of worms, sched_clock() resolution isn't tied to
getnstimeofday(), since you may have cases where the TSC is invalid for
timekeeping (so we use something slow like the ACPI PM) but ok for
scheduling, etc.
Even so, its current_kernel_time() is just tick granularity, so that
probably won't work for you.
So I'm leaning towards Anton's suggestion of adding a new internal accessor
that returns an error if we're suspended.
Thomas, what do you think of something like what's below?
thanks
-john
diff --git a/include/linux/time.h b/include/linux/time.h
index 4d358e9..0015aea 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval
*value,
struct itimerval *ovalue);
extern unsigned int alarm_setitimer(unsigned int seconds);
extern int do_getitimer(int which, struct itimerval *value);
+extern int __getnstimeofday(struct timespec *tv);
extern void getnstimeofday(struct timespec *tv);
extern void getrawmonotonic(struct timespec *ts);
extern void getnstime_raw_and_real(struct timespec *ts_raw,
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..bb2638c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -220,19 +220,20 @@ static void timekeeping_forward_now(struct timekeeper
*tk)
}
/**
- * getnstimeofday - Returns the time of day in a timespec
+ * __getnstimeofday - Returns the time of day in a timespec
* @ts: pointer to the timespec to be set
*
- * Returns the time of day in a timespec.
+ * Returns -1 if timekeeping is suspended.
+ * Returns 0 on success.
*/
-void getnstimeofday(struct timespec *ts)
+int __getnstimeofday(struct timespec *ts)
{
struct timekeeper *tk = &timekeeper;
unsigned long seq;
s64 nsecs = 0;
- WARN_ON(timekeeping_suspended);
-
+ if (unlikely(timekeeping_suspended));
+ return -1;
Is it useful to make this -EAGAIN or something, just for clarity?
Also, this technically changes the semantics of callers that were
hitting WARN (there should be none) in that the timespec isn't updated
at all. In the prior code, a WARN would be emitted, but it would still
fill out the timespec and return.
When I looked at implementing this error path, I actually moved the
return -EAGAIN to the end of the function to keep the results the
same. All that said, this is much cleaner and more correct if not
touching the timespec on error is tolerable.
Yea, although really, the value returned if the WARN_ON is emitted is
really junk. If the clocksource was reset under us, you could see really
crazy time values.
Thinking some more, a better solution could be to simply return the time
when timekeeping was suspended, which would be sane.
This wouldn't catch cases where folks were accessing the time while
timekeeping is suspended, but they would get as valid a value as we can
provide.
The only downside, is that accessors could see a flat-spot in time where
they just get the same value over and over while timekeeping is
suspended. So could be possible for bad behavior if something was doing
something dumb like spinning in a loop waiting for some time to pass and
was preventing suspend from completing.
Untested patch below. Any thoughts?
thanks
-john
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..84593ef 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -146,6 +146,9 @@ static inline s64 timekeeping_get_ns(struct timekeeper *tk)
struct clocksource *clock;
s64 nsec;
+ if (unlikely(timekeeping_suspended))
+ return tk->xtime_nsec >> tk->shift;
+
/* read clocksource: */
clock = tk->clock;
cycle_now = clock->read(clock);
@@ -166,6 +169,9 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk)
struct clocksource *clock;
s64 nsec;
+ if (unlikely(timekeeping_suspended))
+ return 0;
+
/* read clocksource: */
clock = tk->clock;
cycle_now = clock->read(clock);
@@ -231,8 +237,6 @@ void getnstimeofday(struct timespec *ts)
unsigned long seq;
s64 nsecs = 0;
- WARN_ON(timekeeping_suspended);
-
do {
seq = read_seqbegin(&tk->lock);
@@ -252,8 +256,6 @@ ktime_t ktime_get(void)
unsigned int seq;
s64 secs, nsecs;
- WARN_ON(timekeeping_suspended);
-
do {
seq = read_seqbegin(&tk->lock);
secs = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
@@ -283,8 +285,6 @@ void ktime_get_ts(struct timespec *ts)
s64 nsec;
unsigned int seq;
- WARN_ON(timekeeping_suspended);
-
do {
seq = read_seqbegin(&tk->lock);
ts->tv_sec = tk->xtime_sec;
@@ -316,8 +316,6 @@ void getnstime_raw_and_real(struct timespec *ts_raw, struct timespec *ts_real)
unsigned long seq;
s64 nsecs_raw, nsecs_real;
- WARN_ON_ONCE(timekeeping_suspended);
-
do {
seq = read_seqbegin(&tk->lock);
@@ -1203,8 +1201,6 @@ void get_monotonic_boottime(struct timespec *ts)
s64 nsec;
unsigned int seq;
- WARN_ON(timekeeping_suspended);
-
do {
seq = read_seqbegin(&tk->lock);
ts->tv_sec = tk->xtime_sec;
--
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