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

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Date:	Tue, 04 Sep 2012 17:40:08 +0100
From:	Pedro Alves <palves@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Tejun Heo <tj@...nel.org>, torvalds@...ux-foundation.org,
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Subject: Re: [PATCH v3 01/17] hashtable: introduce a small and naive hashtable

On 09/04/2012 05:30 PM, Pedro Alves wrote:
> On 09/04/2012 04:35 PM, Steven Rostedt wrote:
>> On Tue, 2012-08-28 at 19:00 -0400, Mathieu Desnoyers wrote:
>>
>>> Looking again at:
>>>
>>> +#define hash_for_each_size(name, bits, bkt, node, obj, member)                 \
>>> +       for (bkt = 0; bkt < HASH_SIZE(bits); bkt++)                             \
>>> +               hlist_for_each_entry(obj, node, &name[bkt], member)
>>>
>>> you will notice that a "break" or "continue" in the inner loop will not
>>> affect the outer loop, which is certainly not what the programmer would
>>> expect!
>>>
>>> I advise strongly against creating such error-prone construct.
>>>
>>
>> A few existing loop macros do this. But they require a do { } while ()
>> approach, and all have a comment.
>>
>> It's used by do_each_thread() in sched.h and ftrace does this as well.
>> Look at kernel/trace/ftrace.c at do_for_each_ftrace_rec().
>>
>> Yes it breaks 'break' but it does not break 'continue' as it would just
>> go to the next item that would have been found (like a normal for
>> would).
> 
> /*
>  * This is a double for. Do not use 'break' to break out of the loop,
>  * you must use a goto.
>  */
> #define do_for_each_ftrace_rec(pg, rec)                                 \
>         for (pg = ftrace_pages_start; pg; pg = pg->next) {              \
>                 int _____i;                                             \
>                 for (_____i = 0; _____i < pg->index; _____i++) {        \
>                         rec = &pg->records[_____i];
> 
> 
> 
> You can make 'break' also work as expected if you can embed a little knowledge
> of the inner loop's condition in the outer loop's condition.  Sometimes it's
> trivial, most often when the inner loop's iterator is a pointer that goes
> NULL at the end, but other times not so much.  Something like (completely untested):
> 
> #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) {                                           \
>                 int _____i;                                             \
>                 for (_____i = 0; _____i < pg->index; _____i++) {        \
>                         rec = &pg->records[_____i];
>
> 
> (other variants possible)
> 
> IOW, the outer loop only iterates if the inner loop completes.  If there's
> a break in the inner loop, then the outer loop breaks too.  Of course, it
> all depends on whether the generated code looks sane or hideous, if
> the uses of the macro care for it over bug avoidance.
> 

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++)

-- 
Pedro Alves
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