Messages in this thread | | | Date | Thu, 30 Apr 2009 15:37:57 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping around |
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* Nikanth Karthikesan <knikanth@novell.com> wrote:
> > Then there could be a single, straightforward value check: > > > > static inline void atomic_inc(atomic_t *v) > > { > > debug_atomic_check_value(v); > > raw_atomic_inc(v); > > } > > > > Where debug_atomic_check_value() is just an atomic_read(): > > > > static inline void debug_atomic_check_value(atomic_t *v) > > { > > WARN_ONCE(in_range(atomic_read(v), UINT_MAX/4, UINT_MAX/4*3), > > KERN_ERR "atomic counter check failure!"); > > } > > > > I do not understand, why UINT_MAX/4 to UINT_MAX/4*3? > Roughly, > UINT_MAX/4 = INT_MAX/2 > UINT_MAX/4*3 = INT_MAX/2*3 which we will never reach with an int.
i mean:
WARN_ONCE(in_range((u32)atomic_read(v), UINT_MAX/4, UINT_MAX/4*3), KERN_ERR "atomic counter check failure!");
that's a single range check on an u32, selecting 'too large' and 'too small' s32 values.
> > It's a constant check. > > > > If are overflowing on such a massive rate, it doesnt matter how > > early or late we check the value. > > UINT_MAX/4 early, might be too early. And if it doesn't matter how > early or late, why try to be over-cautious and produce false > warnings. ;-)
UINT_MAX/4 is ~1 billion. If we reach a value of 1 billion we are leaking. Your check basically is a sharp test for the specific case of overflowing the boundary - but it makes the code slower (it uses more complex atomic ops) and uglifies it via #ifdefs as well.
It doesnt matter whether we wrap over at around +2 billion into -2 billion, or treat the whole above-1-billion and below-minus-1-billion range as invalid. (other than we'll catch bugs sooner via this method, and have faster and cleaner code)
Ingo
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