> On Tue, 2008-07-15 at 10:27 -0400, Mathieu Desnoyers wrote:
> > * Peter Zijlstra (
peterz@infradead.org) wrote:
> > > On Tue, 2008-07-15 at 09:25 -0400, Mathieu Desnoyers wrote:
> > > > * Peter Zijlstra (
peterz@infradead.org) wrote:
> > > > > On Wed, 2008-07-09 at 10:59 -0400, Mathieu Desnoyers wrote:
> > >
> > > > > > +#define __DO_TRACE(tp, proto, args) \
> > > > > > + do { \
> > > > > > + int i; \
> > > > > > + void **funcs; \
> > > > > > + preempt_disable(); \
> > > > > > + funcs = (tp)->funcs; \
> > > > > > + smp_read_barrier_depends(); \
> > > > > > + if (funcs) { \
> > > > > > + for (i = 0; funcs[i]; i++) { \
> > > > >
> > > > > can't you get rid of 'i' and write:
> > > > >
> > > > > void **func;
> > > > >
> > > > > preempt_disable();
> > > > > func = (tp)->funcs;
> > > > > smp_read_barrier_depends();
> > > > > for (; func; func++)
> > > > > ((void (*)(proto))func)(args);
> > > > > preempt_enable();
> > > > >
> > > >
> > > > Yes, I though there would be an optimization to do here, I'll use your
> > > > proposal. This code snippet is especially important since it will
> > > > generate instructions near every tracepoint side. Saving a few bytes
> > > > becomes important.
> > > >
> > > > Given that (tp)->funcs references an array of function pointers and that
> > > > it can be NULL, the if (funcs) test must still be there and we must use
> > > >
> > > > #define __DO_TRACE(tp, proto, args) \
> > > > do { \
> > > > void *func; \
> > > > \
> > > > preempt_disable(); \
> > > > if ((tp)->funcs) { \
> > > > func = rcu_dereference((tp)->funcs); \
> > > > for (; func; func++) { \
> > > > ((void(*)(proto))(func))(args); \
> > > > } \
> > > > } \
> > > > preempt_enable(); \
> > > > } while (0)
> > > >
> > > >
> > > > The resulting assembly is a bit more dense than my previous
> > > > implementation, which is good :
> > >
> > > My version also has that if ((tp)->funcs), but its hidden in the
> > > for (; func; func++) loop. The only thing your version does is an extra
> > > test of tp->funcs but without read depends barrier - not sure if that is
> > > ok.
> > >
> >
> > Hrm, you are right, the implementation I just proposed is bogus. (but so
> > was yours) ;)
> >
> > func is an iterator on the funcs array. My typing of func is thus wrong,
> > it should be void **. Otherwise I'm just incrementing the function
> > address which is plain wrong.
> >
> > The read barrier is included in rcu_dereference() now. But given that we
> > have to take a pointer to the array as an iterator, we would have to
> > rcu_dereference() our iterator multiple times and then have many read
> > barrier depends, which we don't need. This is why I would go back to a
> > smp_read_barrier_depends().
> >
> > Also, I use a NULL entry at the end of the funcs array as an end of
> > array identifier. However, I cannot use this in the for loop both as a
> > check for NULL array and check for NULL array element. This is why a if
> > () test is needed in addition to the for loop test. (this is actually
> > what is wrong in the implementation you proposed : you treat func both
> > as a pointer to the function pointer array and as a function pointer)
>
> Ah, D'0h! Indeed.
>
> > Something like this seems better :
> >
> > #define __DO_TRACE(tp, proto, args) \
> > do { \
> > void **it_func; \
> > \
> > preempt_disable(); \
> > it_func = (tp)->funcs; \
> > if (it_func) { \
> > smp_read_barrier_depends(); \
> > for (; *it_func; it_func++) \
> > ((void(*)(proto))(*it_func))(args); \
> > } \
> > preempt_enable(); \
> > } while (0)
> >
> > What do you think ?
>
> I'm confused by the barrier games here.
>
> Why not:
>
> void **it_func;
>
> preempt_disable();
> it_func = rcu_dereference((tp)->funcs);
> if (it_func) {
> for (; *it_func; it_func++)
> ((void(*)(proto))(*it_func))(args);
> }
> preempt_enable();
>
> That is, why can we skip the barrier when !it_func? is that because at
> that time we don't actually dereference it_func and therefore cannot
> observe stale data?
>
