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From: Josh Triplett on 26 Aug 2008 20:40
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


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From: Paul E. McKenney on 27 Aug 2008 14:30
On Mon, Aug 25, 2008 at 05:26:43PM +0200, Peter Zijlstra wrote:
> On Mon, 2008-08-25 at 08:16 -0700, Paul E. McKenney wrote:
> > On Mon, Aug 25, 2008 at 12:34:56PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2008-08-22 at 16:29 -0700, Josh Triplett wrote:
> > >
> > > > > @@ -26,8 +27,10 @@
> > > > > * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
> > > > > *
> > > > > * For detailed explanation of Read-Copy Update mechanism see -
> > > > > - * Documentation/RCU
> > > > > - *
> > > > > + * Documentation/RCU
> > > > > + * http://lwn.net/Articles/262464/ (What is RCU, Fundamentally?)
> > > > > + * http://lwn.net/Articles/263130/ (What is RCU's Usage?)
> > > > > + * http://lwn.net/Articles/264090/ (What is RCU's API? + references)
> > > > > */
> > > >
> > > > Why put these references here rather than in Documentation/RCU? It
> > > > seems easier to keep documentation up to date in one place. If you
> > > > think these represent a good "getting started" set of documents, how
> > > > about a Documentation/RCU/ReadTheseFirst with links to them, or how
> > > > about linking to them from whatisRCU.txt?
> > >
> > > I actually like in code comments and 'documentation' more than
> > > Documentation/ stuff. Mostly because Documentation/ is:
> > > - far away from the code
> > > - therefore, more easily bitrotted
> > > - and easily forgotten
> >
> > I know!!!
> >
> > #ifdef JOSH_TRIPLETT
> > * Documentation/RCU
> > * http://lwn.net/Articles/262464/ (What is RCU, Fundamentally?)
> > * http://lwn.net/Articles/263130/ (What is RCU's Usage?)
> > * http://lwn.net/Articles/264090/ (What is RCU's API? + references)
> > #elif PETER_ZIJLSTRA
> > * Documentation/RCU
> > #endif
> >
> > (Sorry, couldn't resist!!!)
>
> But but but, you got the cases the wrong way around.. ;-)

Good point...

#ifdef READER_LIKES_DOCUMENTATION_URLS_IN_COMMENTS
* Documentation/RCU
* http://lwn.net/Articles/262464/ (What is RCU, Fundamentally?)
* http://lwn.net/Articles/263130/ (What is RCU's Usage?)
* http://lwn.net/Articles/264090/ (What is RCU's API? + references)
#else
* Documentation/RCU
#endif

Of course, the C preprocessor would just remove the whole comment
anyway, but hopefully it is the thought that counts. ;-)

> > Seriously, I know where all the documentation is, as I wrote most of it.
> > These comments are for you guys. So, any thoughts on how I should
> > resolve this? My default is, as always, a coin flip. ;-)
>
> I guess we could do the 'this is how the concept works and can be used
> like so and so' documentation in Documentation/

Documentation/RCU/whatisRCU.txt does in fact contain the three URLs
listed above. And there is always Documentation/RCU/RTFP.txt for
people wanting the full effect.

> And the stuff that says 'this code does like so and so, because blah'
> should stay near the code.
>
> And in any case of doubt - stay near the code :-)
>
> I always view Documentation/ as end user stuff (be that a kernel
> programmer that needs to learn a new API, or userland folks or people
> wanting to know what a certain feature is about).

I confess to erring on the side of spamming all channels. Then again,
I am a serial junk-mailer, so perhaps this is just me.

Thanx, Paul
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From: Paul E. McKenney on 27 Aug 2008 14:40
On Tue, Aug 26, 2008 at 05:38:36PM -0700, Josh Triplett wrote:
> 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.

Even if one argument of || is long and the other int or some fool thing
like that? (What, me paranoid???)

Thanx, Paul
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From: Josh Triplett on 27 Aug 2008 16:30
On Wed, 2008-08-27 at 11:34 -0700, Paul E. McKenney wrote:
> On Tue, Aug 26, 2008 at 05:38:36PM -0700, Josh Triplett wrote:
> > 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.
>
> Even if one argument of || is long and the other int or some fool thing
> like that? (What, me paranoid???)

What, you don't know exactly how C behaves in every strange corner
case? ;)

|| always produces a result of type int, and it compares each of its two
arguments to 0 independently; to the best of my knowledge the size of
those arguments never matters.

- Josh Triplett


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From: Paul E. McKenney on 27 Aug 2008 16:50
On Wed, Aug 27, 2008 at 01:23:28PM -0700, Josh Triplett wrote:
> On Wed, 2008-08-27 at 11:34 -0700, Paul E. McKenney wrote:
> > On Tue, Aug 26, 2008 at 05:38:36PM -0700, Josh Triplett wrote:
> > > 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.
> >
> > Even if one argument of || is long and the other int or some fool thing
> > like that? (What, me paranoid???)
>
> What, you don't know exactly how C behaves in every strange corner
> case? ;)

I used to, back when identifiers were only guaranteed to be
differentiated by their first 8 characters (6 or 7 if extern). ;-)

> || always produces a result of type int, and it compares each of its two
> arguments to 0 independently; to the best of my knowledge the size of
> those arguments never matters.

I suppose I should read the spec.

Thanx, Paul
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