Messages in this thread | | | Date | Thu, 8 Mar 2012 11:49:05 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call_srcu() |
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On Tue, Mar 06, 2012 at 04:38:18PM +0100, Peter Zijlstra wrote: > On Tue, 2012-03-06 at 23:17 +0800, Lai Jiangshan wrote: > > On Tue, Mar 6, 2012 at 6:58 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > > On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote: > > >> /* > > >> + * 'return left < right;' but handle the overflow issues. > > >> + * The same as 'return (long)(right - left) > 0;' but it cares more. > > > > > > About what? And why? We do the (long)(a - b) thing all over the kernel, > > > why would you care more? > > > > @left is constants of the callers(callbacks's snapshot), @right > > increases very slow. > > if (long)(right - left) is a big negative, we have to wait for a long > > time in this kinds of overflow. > > this kinds of overflow can not happen in this safe_less_than() > > I'm afraid I'm being particularly dense, but what?!
I have been converting the "(long)(a - b)" stuff in RCU to use unsigned arithmetic. The ULONG_CMP_GE() and friends in rcupdate.h are for this purpose.
I too have used (long)(a - b) for a long time, but I saw with my own eyes the glee in the compiler-writers' eyes when they discussed signed overflow being undefined in the C standard. I believe that the reasons for signed overflow being undefined are long obsolete, but better safe than sorry.
Thanx, Paul
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