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    SubjectRe: [PATCH] debug rcu head support !PREEMPT config
    * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
    > On Wed, Feb 23, 2011 at 10:37:26AM -0500, Mathieu Desnoyers wrote:
    > > * Steven Rostedt (rostedt@goodmis.org) wrote:
    > > > On Wed, 2011-02-23 at 10:13 -0500, Mathieu Desnoyers wrote:
    > > > > Remove DEBUG_RCU_HEAD dependency on PREEMPT config. Handle the unability to
    > > > > detect if within a RCU read-side critical section by never performing any
    > > > > attempt to recover from a failure situation in the fixup handlers. Just print
    > > > > the warnings.
    > > > >
    > > > > This patch is only compile-tested.
    > > > >
    > > > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    > > > > ---
    > > > > kernel/rcupdate.c | 17 +++++++++++++++++
    > > > > lib/Kconfig.debug | 2 +-
    > > > > 2 files changed, 18 insertions(+), 1 deletion(-)
    > > > >
    > > > > Index: linux-2.6-lttng/lib/Kconfig.debug
    > > > > ===================================================================
    > > > > --- linux-2.6-lttng.orig/lib/Kconfig.debug
    > > > > +++ linux-2.6-lttng/lib/Kconfig.debug
    > > > > @@ -313,7 +313,7 @@ config DEBUG_OBJECTS_WORK
    > > > >
    > > > > config DEBUG_OBJECTS_RCU_HEAD
    > > > > bool "Debug RCU callbacks objects"
    > > > > - depends on DEBUG_OBJECTS && PREEMPT
    > > > > + depends on DEBUG_OBJECTS
    > > > > help
    > > > > Enable this to turn on debugging of RCU list heads (call_rcu() usage).
    > > > >
    > > > > Index: linux-2.6-lttng/kernel/rcupdate.c
    > > > > ===================================================================
    > > > > --- linux-2.6-lttng.orig/kernel/rcupdate.c
    > > > > +++ linux-2.6-lttng/kernel/rcupdate.c
    > > > > @@ -142,7 +142,14 @@ static int rcuhead_fixup_init(void *addr
    > > > > * 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.
    > > > > + * In !PREEMPT configurations, there is no way to tell if we are
    > > > > + * in a RCU read-side critical section or not, so we never
    > > > > + * attempt any fixup and just print a warning.
    > > > > */
    > > > > +#ifndef CONFIG_PREEMPT
    > > > > + WARN_ON(1);
    > > > > + return 0;
    > > > > +#endif
    > > > > if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
    > > > > irqs_disabled()) {
    > > > > WARN_ON(1);
    > > > > @@ -184,7 +191,14 @@ static int rcuhead_fixup_activate(void *
    > > > > * 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.
    > > > > + * In !PREEMPT configurations, there is no way to tell if we are
    > > > > + * in a RCU read-side critical section or not, so we never
    > > > > + * attempt any fixup and just print a warning.
    > > > > */
    > > > > +#ifndef CONFIG_PREEMPT
    > > > > + WARN_ON(1);
    > > > > + return 0;
    > > > > +#endif
    > > > > if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
    > > > > irqs_disabled()) {
    > > > > WARN_ON(1);
    > > > > @@ -214,6 +228,9 @@ static int rcuhead_fixup_free(void *addr
    > > > > * 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.
    > > > > + * In !PREEMPT configurations, there is no way to tell if we are
    > > > > + * in a RCU read-side critical section or not, so we never
    > > > > + * attempt any fixup and just print a warning.
    > > > > */
    > > > > #ifndef CONFIG_PREEMPT
    > > > > WARN_ON(1);
    > > >
    > > > Hmm, I wonder if s/WARN_ON/WARN_ON_ONCE/g is in order. Why spam the
    > > > console if it happens to trigger all the time?
    > >
    > > The system should die pretty soon anyway due to list corruption, so I
    > > don't think it's a problem in practice.
    >
    > Well, it would add noise, so I added a patch converting the WARN_ON's
    > to WARN_ON_ONCE's.

    Thinking about it, it makes a lot of sense. Thanks!

    Mathieu


    --
    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
    http://www.efficios.com


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