Messages in this thread Patch in this message | | | From | Pavel Ivanov <> | Date | Mon, 26 Sep 2011 18:04:06 -0400 | Subject | Re: [PATCH 1/7] rcu: Fix preempt-unsafe debug check of rcu extended quiescent state |
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On Mon, Sep 26, 2011 at 6:19 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote: > In the rcu_check_extended_qs() function that is used to check > illegal uses of RCU under extended quiescent states, we look > at the local value of dynticks that is even if an extended > quiescent state or odd otherwise. > > We are looking at it without disabling the preemption though > and this opens a race window where we may read the state of > a remote CPU instead, like in the following scenario: > > CPU 1 CPU 2 > > bool rcu_check_extended_qs(void) > { > struct rcu_dynticks *rdtp; > > rdtp = &per_cpu(rcu_dynticks, > raw_smp_processor_id()); > > < ---- Task is migrated here ---- > > > // CPU 1 goes idle and increase rdtp->dynticks > /* Here we are reading the value for > the remote CPU 1 instead of the local CPU */ > if (atomic_read(&rdtp->dynticks) & 0x1) > return false; > > return true; > } > > The possible result of this is false positive return value of that > function, suggesting we are in an extended quiescent state in random > places.
How is this different from what your patch allows?
CPU 1 CPU 2
bool rcu_check_extended_qs(void) { struct rcu_dynticks *rdtp = &get_cpu_var(rcu_dynticks); bool ext_qs = true;
if (atomic_read(&rdtp->dynticks) & 0x1) ext_qs = false;
put_cpu_var(rcu_dynticks);
< ---- Task is migrated here ---- >
/* Here we return true/false based on the value for the remote CPU 1 instead of the local CPU */
return ext_qs; }
Looks like it can result in the same false positive result.
Pavel
> Fix this by disabling preemption while reading that value. > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com> > Cc: Ingo Molnar <mingo@redhat.com> > --- > kernel/rcutree.c | 10 ++++++---- > 1 files changed, 6 insertions(+), 4 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index c9b4adf..234dca3 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -469,13 +469,15 @@ void rcu_irq_exit(void) > > bool rcu_check_extended_qs(void) > { > - struct rcu_dynticks *rdtp; > + struct rcu_dynticks *rdtp = &get_cpu_var(rcu_dynticks); > + bool ext_qs = true; > > - rdtp = &per_cpu(rcu_dynticks, raw_smp_processor_id()); > if (atomic_read(&rdtp->dynticks) & 0x1) > - return false; > + ext_qs = false; > + > + put_cpu_var(rcu_dynticks); > > - return true; > + return ext_qs; > } > EXPORT_SYMBOL_GPL(rcu_check_extended_qs); > > -- > 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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