Re: [dm-devel] [PATCH v7 06/16] tracepoint: use new hashtable implementation

[Sasha Levin <levinsasha928 gmail com>]

On Mon, Oct 29, 2012 at 7:35 AM, Mathieu Desnoyers
<mathieu desnoyers efficios com> wrote:
> * Sasha Levin (levinsasha928 gmail com) wrote:
>> Switch tracepoints to use the new hashtable implementation. This reduces the amount of
>> generic unrelated code in the tracepoints.
>>
>> Signed-off-by: Sasha Levin <levinsasha928 gmail com>
>> ---
>>  kernel/tracepoint.c | 27 +++++++++++----------------
>>  1 file changed, 11 insertions(+), 16 deletions(-)
>>
>> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
>> index d96ba22..854df92 100644
>> --- a/kernel/tracepoint.c
>> +++ b/kernel/tracepoint.c
>> @@ -26,6 +26,7 @@
>>  #include <linux/slab.h>
>>  #include <linux/sched.h>
>>  #include <linux/static_key.h>
>> +#include <linux/hashtable.h>
>>
>>  extern struct tracepoint * const __start___tracepoints_ptrs[];
>>  extern struct tracepoint * const __stop___tracepoints_ptrs[];
>> @@ -49,8 +50,7 @@ static LIST_HEAD(tracepoint_module_list);
>>   * Protected by tracepoints_mutex.
>>   */
>>  #define TRACEPOINT_HASH_BITS 6
>> -#define TRACEPOINT_TABLE_SIZE (1 << TRACEPOINT_HASH_BITS)
>> -static struct hlist_head tracepoint_table[TRACEPOINT_TABLE_SIZE];
>> +static DEFINE_HASHTABLE(tracepoint_table, TRACEPOINT_HASH_BITS);
>>
> [...]
>>
>> @@ -722,6 +715,8 @@ struct notifier_block tracepoint_module_nb = {
>>
>>  static int init_tracepoints(void)
>>  {
>> +     hash_init(tracepoint_table);
>> +
>>       return register_module_notifier(&tracepoint_module_nb);
>>  }
>>  __initcall(init_tracepoints);
>
> So we have a hash table defined in .bss (therefore entirely initialized
> to NULL), and you add a call to "hash_init", which iterates on the whole
> array and initialize it to NULL (again) ?
>
> This extra initialization is redundant. I think it should be removed
> from here, and hashtable.h should document that hash_init() don't need
> to be called on zeroed memory (which includes static/global variables,
> kzalloc'd memory, etc).

This was discussed in the previous series, the conclusion was to call
hash_init() either way to keep the encapsulation and consistency.

It's cheap enough and happens only once, so why not?


Thanks,
Sasha