lkml.org 
[lkml]   [2011]   [Feb]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
    From
    Date
    On Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote:
    > * Steven Rostedt (rostedt@goodmis.org) wrote:
    > > On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote:
    > > > The build will break if you change the Kconfig to allow
    > > > DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
    > > > near where the breakage would occur.
    > > >
    > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > > ---
    > > > kernel/rcupdate.c | 5 +++++
    > > > 1 files changed, 5 insertions(+), 0 deletions(-)
    > > >
    > > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
    > > > index afd21d1..f3240e9 100644
    > > > --- a/kernel/rcupdate.c
    > > > +++ b/kernel/rcupdate.c
    > > > @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
    > > > * Ensure that queued callbacks are all executed.
    > > > * If we detect that we are nested in a RCU read-side critical
    > > > * section, we should simply fail, otherwise we would deadlock.
    > > > + * Note that the machinery to reliably determine whether
    > > > + * or not we are in an RCU read-side critical section
    > > > + * exists only in the preemptible RCU implementations
    > > > + * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
    > > > + * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
    > > > */
    > >
    > > Shouldn't this comment also be in the kconfig where
    > > DEBUG_OBJECTS_RCU_HEAD is defined?
    >
    > hrm, but this is a "rcuhead_fixup_init" : it does not need to always

    Looks like it was in rcuhead_fixup_free to me.

    > succeed. It's just that when it is safe to recover from an error
    > situation, it does it. We could do:
    >
    > #ifdef CONFIG_PREEMPT
    > /*
    > * Ensure that queued callbacks are all executed.
    > * If we detect that we are nested in a RCU read-side
    > * critical
    > * section, we should simply fail, otherwise we would
    > * deadlock.
    > */
    > if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
    > irqs_disabled()) {
    > WARN_ON(1);
    > return 0;
    > }
    > rcu_barrier();
    > rcu_barrier_sched();
    > rcu_barrier_bh();
    > debug_object_init(head, &rcuhead_debug_descr);
    > return 1;
    > #else
    > return 0;
    > #endif
    >
    > instead, no ?

    The point is that this entire block of code is wrapped in #ifdef
    CONFIG_DEBUG_OBJECTS_RCU_HEAD, and that config depends on PREEMPT. Thus
    you will never have a case where #ifdef CONFIG_PREEMPT is false.

    -- Steve




    \
     
     \ /
      Last update: 2011-02-23 15:13    [W:0.023 / U:0.332 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site