Messages in this thread | | | Subject | Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation | From | Josh Triplett <> | Date | Tue, 26 Aug 2008 17:38:36 -0700 |
| |
On Tue, 2008-08-26 at 09:05 -0700, Paul E. McKenney wrote: > On Mon, Aug 25, 2008 at 03:02:30PM -0700, Josh Triplett wrote: > > On Fri, 2008-08-22 at 18:53 -0700, Paul E. McKenney wrote: > > > On Fri, Aug 22, 2008 at 04:29:32PM -0700, Josh Triplett wrote: > > > > On Thu, 2008-08-21 at 16:43 -0700, Paul E. McKenney wrote: > > > > > @@ -658,14 +806,19 @@ int rcu_needs_cpu(int cpu) > > > > > struct rcu_data *rdp = &per_cpu(rcu_data, cpu); > > > > > struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu); > > > > > > > > > > - return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu); > > > > > + return !!*rdp->nxttail[RCU_DONE_TAIL] || > > > > > + !!*rdp_bh->nxttail[RCU_DONE_TAIL] || > > > > > + rcu_pending(cpu); > > > > > > > > !! seems unnecessary here. > > > > > > Someone once told me why this was necessary, but I forget. It was in the > > > original, and I didn't put it there. Some weirdness about conversion > > > to 32-bit integer when the lower 32 bits of the pointer was zero or > > > some such. So if your pointer value was 0x100000000, for example, > > > so that conversion to int gives zero. > > > > Good point! That doesn't apply if you use ||, though. If you just did > > "return somepointer" that could potentially cause the problem you > > describe. In any case, it can't *hurt* to have it; GCC should do the > > sane thing. > > OK. I will review this towards the end, leaving it there to remind me > in the meantime. > > So, would I need the !! on the left-hand operand of the first || due > to short-circuiting?
No. || will always return 1 or 0. You only need the !! if you want to directly return the boolean value of a potentially 64-bit pointer.
- Josh Triplett
| |