Messages in this thread | | | Date | Sun, 24 May 2015 20:18:31 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] locking: type cleanup when accessing fast_read_ctr |
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On 05/23, Nicholas Mc Guire wrote: > > On Wed, 20 May 2015, Oleg Nesterov wrote: > > > On 05/19, Nicholas Mc Guire wrote: > > > > > > I assumed it would not matter but did not see a simple way of getting it > > > type clean with unsigned either mainly due to the atomic_t being int and > > > val in update_fast_ctr() being passed as -1. > > > > Perhaps clear_fast_ctr() should have a comment to explain why it returns > > "int"... even if "unsigned" should work too. > > > Might not be into c99 standard far enough but from reviewing 6.5/J.2 > I do not see this argument here. > > The "well defined modulo 2**n" behavior for unsigned int can be > found stated in a few places - but not in the c99 standard for > arithmetic overflow. > > The "well defined overflow behavior" as far as I understand c99, > *only* applies to left shift operations when overflowing - see 6.5.7 " > Bitwise shift operators" -> Semantics -> 4) - further for the counter > problem this well defined behavior is of little help as the sum would > be wrong in both cases. > > I still do not see the point in the implicit/automatic type conversion > here and why that should be an advantage - could somone point me to > the right c99 clauses ?
Sorry, I don't really understand your question...
Once again. Signed overflow is undefined behaviour according to C standard. That is why ->fast_read_ctr is "unsigned long", this counter can actually over/underflow if down/up happens on different CPU's.
clear_fast_ctr() returns "signed int" just because this looks better to me, it can actually return the negative number. If you make it return "unsigned" you will simply shift this implicit/automatic type conversion to atomic_add() which accepts "int i".
Let me also quote Linus:
Now, I doubt you'll find an architecture or C compiler where this will actually ever make a difference,
Yes. So this all is actually cosmetic.
Oleg.
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