Messages in this thread | | | Date | Tue, 8 Apr 2014 20:56:13 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH] time: Fix truncation in jiffies_to_usecs() |
| |
On Tue, Apr 08, 2014 at 02:15:43PM -0400, Steven Rostedt wrote: > On Tue, 8 Apr 2014 19:49:51 +0200 > Frederic Weisbecker <fweisbec@gmail.com> wrote: > > > On Mon, Apr 07, 2014 at 10:34:51PM -0700, Tony Luck wrote: > > > On Mon, Apr 7, 2014 at 3:25 PM, Tony Luck <tony.luck@intel.com> wrote: > > > > > > > c) If not this ... then what? Separate routine to convert large numbers > > > > of jiffies to usec/nsecs? Should we make the existing one barf when > > > > handed a number that overflows? > > > > > > Having thought about this a bit more - I'm leaning towards leaving > > > jiffies_to_usecs() alone, but using it as a model for a from-scratch > > > implementation of: > > > u64 jiffies_to_nsecs(const unsigned long j) > > > { > > > } > > > > > > This is what the uptime tracer actually needs - and there is only > > > one user of jiffies_to_nsecs() to worry about. > > > > I'm not sure I get what you're trying to do. We already have jiffies_to_nsecs(). > > Anyway I'll just wait and check out the next patch :) > > I believe the issue is the way it's implemented: > > static inline u64 jiffies_to_nsecs(const unsigned long j) > { > return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC; > } > > The problem is with jiffies_to_usecs(). Which we probably should > change. > > With HZ = 100, > 1 second jiffies_to_usecs(100) = 1000,000. > 1 minute jiffies_to_usec(6000) = 60,000,000. > 1 hour jiffies_to_usecs(360000) = 3,600,000,000 > 1 hour 11 minutes 35 seconds - > jiffies_to_usecs(429500) = 4,295,000,000 > > 2^32 = 4294967296 < 4,295,000,000 > > Overflow! > > That means after 1 hour, 11 minutes and 35 seconds, jiffies_to_usecs() > will return a reset number. Time will go backwards. It doesn't matter > what you typecast the return value of jiffies_to_usecs() to, the result > is wrong.
Ah! Ok got it now.
> Actually, I like Tony's first patch. I really think jiffies_to_usecs() > should return a u64 number.
Agreed it's way too error-prone. OTOH there are too many users to allow such a blind broad conversion of its return type:
$ git grep jiffies_to_usecs | cut -f1 | wc -l 52
So it may indeed be a better idea to first create a standalone jiffies_to_nsecs(). It can then be used to deprecate and replace most (if not all) calls to jiffies_to_usecs() altogether. Just the conversion must be made one by one to make sure that users can handle that.
Of course a big fat comment on jiffies_to_usecs() to describe that it's unsafe and deprecated would help a bit.
| |