Message-ID: <50AA8104.6070108@us.ibm.com>
Date:	Mon, 19 Nov 2012 10:57:08 -0800
From:	John Stultz <johnstul@...ibm.com>
To:	Kees Cook <keescook@...omium.org>
CC:	Anton Vorontsov <anton.vorontsov@...aro.org>,
	linux-kernel@...r.kernel.org, Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

On 11/19/2012 09:45 AM, Kees Cook wrote:
> On Mon, Nov 19, 2012 at 9:23 AM, John Stultz <johnstul@...ibm.com> wrote:
>> On 11/18/2012 12:09 PM, Kees Cook wrote:
>>> On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@...ibm.com> 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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