On Fri, Jan 08, 2016 at 07:53:41AM -0800, Paul E. McKenney wrote: > On Fri, Jan 08, 2016 at 12:54:53PM +0800, Boqun Feng wrote: > > On Thu, Jan 07, 2016 at 08:48:35PM -0800, Paul E. McKenney wrote: > > > On Fri, Jan 08, 2016 at 03:08:50PM +1100, Stephen Rothwell wrote: > > > > Hi Paul, > > > > > > > > On Thu, 7 Jan 2016 19:41:57 -0800 "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > Indeed, I suspect that SRCU might not be the only thing that would like > > > > > static per-CPU variables. ;-) > > > > > > > > from include/linux/percpu-defs.h: > > > > > > > > * s390 and alpha modules require percpu variables to be defined as > > > > * weak to force the compiler to generate GOT based external > > > > * references for them. This is necessary because percpu sections > > > > * will be located outside of the usually addressable area. > > > > * > > > > * This definition puts the following two extra restrictions when > > > > * defining percpu variables. > > > > * > > > > * 1. The symbol must be globally unique, even the static ones. > > > > * 2. Static percpu variables cannot be defined inside a function. > > > > * > > > > * Archs which need weak percpu definitions should define > > > > * ARCH_NEEDS_WEAK_PER_CPU in asm/percpu.h when necessary. > > > > * > > > > * To ensure that the generic code observes the above two > > > > * restrictions, if CONFIG_DEBUG_FORCE_WEAK_PER_CPU is set weak > > > > * definition is used for all cases. > > > > > > > > so they can be static, but must be globally unique. > > > > Interesting ;-) Thank you. > > > > > Thank you for the tutorial! I have updated the patch to use a unique > > > name for the srcu_struct added by rcuperf.c. > > > > Plus a comment for DEFINE_STATIC_SRCU saying srcu can be defined as > > static but must have a unique name for this reason? > > Yes, plus some words about how to work around the limitation. How about > the patch below? > Looks very good to me ;-) Regards, Boqun > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 61822ff81839ee3c5b6094ce348944d972b87892 > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Date: Fri Jan 8 07:43:50 2016 -0800 > > rcu: Document unique-name limitation for DEFINE_STATIC_SRCU() > > SRCU uses per-CPU variables, and DEFINE_STATIC_SRCU() uses a static > per-CPU variable. However, per-CPU variables have significant > restrictions, for example, names of per-CPU variables must be globally > unique, even if declared static. These restrictions carry over to > DEFINE_STATIC_SRCU(), and this commit therefore documents these > restrictions. > > Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> > Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx> > Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h > index f5f80c5643ac..dc8eb63c6568 100644 > --- a/include/linux/srcu.h > +++ b/include/linux/srcu.h > @@ -99,8 +99,23 @@ void process_srcu(struct work_struct *work); > } > > /* > - * define and init a srcu struct at build time. > - * dont't call init_srcu_struct() nor cleanup_srcu_struct() on it. > + * Define and initialize a srcu struct at build time. > + * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it. > + * > + * Note that although DEFINE_STATIC_SRCU() hides the name from other > + * files, the per-CPU variable rules nevertheless require that the > + * chosen name be globally unique. These rules also prohibit use of > + * DEFINE_STATIC_SRCU() within a function. If these rules are too > + * restrictive, declare the srcu_struct manually. For example, in > + * each file: > + * > + * static struct srcu_struct my_srcu; > + * > + * Then, before the first use of each my_srcu, manually initialize it: > + * > + * init_srcu_struct(&my_srcu); > + * > + * See include/linux/percpu-defs.h for the rules on per-CPU variables. > */ > #define __DEFINE_SRCU(name, is_static) \ > static DEFINE_PER_CPU(struct srcu_struct_array, name##_srcu_array);\ >
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