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

Gaëtan Rivet gaetan.rivet at 6wind.com
Sun Apr 15 17:13:13 CEST 2018


Hello Neil,

On Sat, Apr 14, 2018 at 02:45:45PM -0400, Neil Horman wrote:
> On Fri, Apr 13, 2018 at 02:55:11PM +0200, Gaëtan Rivet wrote:
> > Hi Shreyansh,
> > 
> > On Fri, Apr 13, 2018 at 06:22:43PM +0530, Shreyansh Jain wrote:
> > > 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.
> > 
> > This is interesting, thanks for looking up the GCC code.
> > Ok, unless someone has a strong reason not to, I will add
> > RTE_PRIORITY_LAST. Not really convinced about it but not
> > opposed enough either :) .
> > 
> I concur.  It sounds like gcc is safe, but clangs priority scheme makes me want
> our priorities to be explicit.

Unless I've misunderstood, clang documentation describes the same
behavior as GCC?

Anyway, I've sent a new series with RTE_PRIORITY_LAST:

https://dpdk.org/dev/patchwork/patch/38126/

-- 
Gaëtan Rivet
6WIND


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