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

Pedro Alves
2012-09-04T16:40:56Z

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