[dpdk-dev] [PATCH v5 02/21] eal: list acceptable init priorities

[Shreyansh Jain]

On Friday 13 April 2018 05:12 PM, Neil Horman wrote:
> On Thu, Apr 12, 2018 at 11:57:47PM +0200, Gaëtan Rivet wrote:
>> Hello Neil,
>>
>> On Thu, Apr 12, 2018 at 07:28:26AM -0400, Neil Horman wrote:
>>> On Wed, Apr 11, 2018 at 02:04:03AM +0200, Gaetan Rivet wrote:
>>>> Build a central list to quickly see each used priorities for
>>>> constructors, allowing to verify that they are both above 100 and in the
>>>> proper order.
>>>>
>>>> Signed-off-by: Gaetan Rivet <gaetan.rivet at 6wind.com>
>>>> Acked-by: Neil Horman <nhorman at tuxdriver.com>
>>>> Acked-by: Shreyansh Jain <shreyansh.jain at nxp.com>
>>>> ---
>>>>   lib/librte_eal/common/eal_common_log.c     | 2 +-
>>>>   lib/librte_eal/common/include/rte_bus.h    | 2 +-
>>>>   lib/librte_eal/common/include/rte_common.h | 8 +++++++-
>>>>   3 files changed, 9 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/lib/librte_eal/common/eal_common_log.c b/lib/librte_eal/common/eal_common_log.c
>>>> index a27192620..36b9d6e08 100644
>>>> --- a/lib/librte_eal/common/eal_common_log.c
>>>> +++ b/lib/librte_eal/common/eal_common_log.c
>>>> @@ -260,7 +260,7 @@ static const struct logtype logtype_strings[] = {
>>>>   };
>>>>   
>>>>   /* Logging should be first initializer (before drivers and bus) */
>>>> -RTE_INIT_PRIO(rte_log_init, 101);
>>>> +RTE_INIT_PRIO(rte_log_init, LOG);
>>>>   static void
>>>>   rte_log_init(void)
>>>>   {
>>>> diff --git a/lib/librte_eal/common/include/rte_bus.h b/lib/librte_eal/common/include/rte_bus.h
>>>> index 6fb08341a..eb9eded4e 100644
>>>> --- a/lib/librte_eal/common/include/rte_bus.h
>>>> +++ b/lib/librte_eal/common/include/rte_bus.h
>>>> @@ -325,7 +325,7 @@ enum rte_iova_mode rte_bus_get_iommu_class(void);
>>>>    * The constructor has higher priority than PMD constructors.
>>>>    */
>>>>   #define RTE_REGISTER_BUS(nm, bus) \
>>>> -RTE_INIT_PRIO(businitfn_ ##nm, 110); \
>>>> +RTE_INIT_PRIO(businitfn_ ##nm, BUS); \
>>>>   static void businitfn_ ##nm(void) \
>>>>   {\
>>>>   	(bus).name = RTE_STR(nm);\
>>>> diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
>>>> index 6c5bc5a76..8f04518f7 100644
>>>> --- a/lib/librte_eal/common/include/rte_common.h
>>>> +++ b/lib/librte_eal/common/include/rte_common.h
>>>> @@ -81,6 +81,12 @@ typedef uint16_t unaligned_uint16_t;
>>>>    */
>>>>   #define RTE_SET_USED(x) (void)(x)
>>>>   
>>>> +#define RTE_PRIORITY_LOG 101
>>>> +#define RTE_PRIORITY_BUS 110
>>>> +
>>>> +#define RTE_PRIO(prio) \
>>>> +	RTE_PRIORITY_ ## prio
>>>> +
>>>>   /**
>>>>    * Run function before main() with low priority.
>>>>    *
>>>> @@ -102,7 +108,7 @@ static void __attribute__((constructor, used)) func(void)
>>>>    *   Lowest number is the first to run.
>>>>    */
>>>>   #define RTE_INIT_PRIO(func, prio) \
>>>> -static void __attribute__((constructor(prio), used)) func(void)
>>>> +static void __attribute__((constructor(RTE_PRIO(prio)), used)) func(void)
>>>>   
>>> It just occured to me, that perhaps you should add a RTE_PRORITY_LAST priority,
>>> and redefine RTE_INIT to RTE_INIT_PRIO(func, RTE_PRIORITY_LAST) for clarity.  I
>>> presume that constructors with no explicit priority run last, but the gcc
>>> manual doesn't explicitly say that.  It would be a heck of a bug to track down
>>> if somehow unprioritized constructors ran early.
>>>
>>> Neil
>>>
>>
>> While certainly poorly documented, the behavior is well-defined. I don't see
>> a situation where the bug you describe could arise.
>>
>> Adding RTE_PRIORITY_LAST is pretty harmless, but I'm not sure it's
>> justified to add it. If you still think it is useful, I will do it.
>>
> It was more just a way to unify the macros is all, probably not important.
> 
>> I'd be curious to hear if anyone has had issues of this kind.
>>
> I've not had any, but I was suprised to see that the gcc manual didn't
> explicitly call out the implied priority of unprioritized constructors

I (tried to) looked into the gcc code base. It seems that when priority 
is not defined, DEFAULT_INIT_PRIORITY 65536, is used.

--->8--- gcc/collect2.c ---
   /* Extract init_p number from ctor/dtor name.  */
   pri = atoi (name + pos);
   return pri ? pri : DEFAULT_INIT_PRIORITY;
--->8---

Though, I couldn't find any documentation for this fact - and, I can 
never be confident about gcc code.

I found one of the ARM compiler (clang) does has a policy for using 
non-specified priority as lower than specified priority. [1]

[1] 
https://developer.arm.com/docs/dui0774/latest/compiler-specific-function-variable-and-type-attributes/__attribute__constructorpriority-function-attribute

A specified value for RTE_PRIORITY_LAST is not a bad option - it would 
help in keeping the priorities bound without relying on the unknown of 
priority for unspecified constructors.

> 
> Neil
> 
>> -- 
>> Gaëtan Rivet
>> 6WIND
>>