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Re: [dm-devel] [PATCH v3 01/17] hashtable: introduce a small and naive hashtable



On Tue, 2012-09-04 at 22:51 +0100, Pedro Alves wrote:
> On 09/04/2012 09:59 PM, Steven Rostedt wrote:
> > On Tue, 2012-09-04 at 18:21 +0100, Pedro Alves wrote:
> >> On 09/04/2012 06:17 PM, Steven Rostedt wrote:
> >>> On Tue, 2012-09-04 at 17:40 +0100, Pedro Alves wrote:
> >>>
> >>>> BTW, you can also go a step further and remove the need to close with double }},
> >>>> with something like:
> >>>>
> >>>> #define do_for_each_ftrace_rec(pg, rec)                                          \
> >>>>         for (pg = ftrace_pages_start, rec = &pg->records[pg->index];             \
> >>>>              pg && rec == &pg->records[pg->index];                               \
> >>>>              pg = pg->next)                                                      \
> >>>>           for (rec = pg->records; rec < &pg->records[pg->index]; rec++)
> >>>>
> >>>
> >>> Yeah, but why bother? It's hidden in a macro, and the extra '{ }' shows
> >>> that this is something "special".
> >>
> >> The point of both changes is that there's nothing special in the end
> >> at all.  It all just works...
> >>
> > 
> > It would still fail on a 'break'. The 'while' macro tells us that it is
> > special, because in the end, it wont work.
> 
> Please explain why it would fail on a 'break'.
> 

Ah, I missed the condition with the rec == &pg->records[pg->index]. But
if ftrace_pages_start is NULL, the rec = &pg->records[pg->index] will
fault.

You could do something like rec = pg ? &pg->records[pg->index] : NULL,
but IIRC, the comma operator does not guarantee order evaluation. That
is, the compiler is allowed to process "a , b" as "b; a;" and not "a;
b;".

-- Steve



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