[uml-devel] Re: non-scalar ktime addition and subtraction broken

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 2006-06-01 at 23:08 -0400, Jeff Dike wrote:
> The use of 64-bit additions and subtractions on something which is
> nominally a struct containing 32-bit second and nanosecond field is
> broken when a negative time is involved.  When the structure is
> treated as a 64-bit integer, the increment of the upper 32 bits that's
> part of two's-complement subtraction is lost.  This leaves the end
> result off by one second.
> 
> This manifested itself with sleeps inside UML lasting about 1 second
> shorter than expected.
> 
> The patch below is more a problem statement than a real fix.  People
> thought about performance, and I don't know what this does to that
> work.
> 
> I'm not sure why the hrtimer.c part is needed - I had done that before
> tracking down the ktime_add problem.  I see short sleeps without it,
> so it is needed somehow.
> 
> The ktime_sub piece was done for completeness - UML compiles and boots
> with no apparent ill effects, but it's otherwise untested.
> 
> As an aside, I fail to see how it can be correct for ktime_sub to add
> NSEC_PER_SEC to something without compensating somewhere else for it.
> 
> Andrew - please don't drop this into -mm without an OK from Thomas or
> someone else who's familiar with this code :-)

NAK. ktime_t is defined that ist must be normalized the same way as
timespecs. The nsec part must be >= 0 and < NSEC_PER_SEC. Fix the part
which is feeding non normalized values.

	tglx






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