> Implementing RCU-bh in terms of RCU-preempt makes the system vulnerable > to network-based denial-of-service attacks. This patch therefore > makes __do_softirq() invoke rcu_bh_qs(), but only when __do_softirq() > is running in ksoftirqd context. A wrapper layer in interposed so that > other calls to __do_softirq() avoid invoking rcu_bh_qs(). The underlying > function __do_softirq_common() does the actual work. > > The reason that rcu_bh_qs() is bad in these non-ksoftirqd contexts is > that there might be a local_bh_enable() inside an RCU-preempt read-side > critical section. This local_bh_enable() can invoke __do_softirq() > directly, so if __do_softirq() were to invoke rcu_bh_qs() (which just > calls rcu_preempt_qs() in the PREEMPT_RT_FULL case), there would be > an illegal RCU-preempt quiescent state in the middle of an RCU-preempt > read-side critical section. Therefore, quiescent states can only happen > in cases where __do_softirq() is invoked directly from ksoftirqd. I -think- that the need for this goes away in the current merge window because RCU-bh is going away. There might still be an rt-specific need to disable irqs, though. Thanx, Paul > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Link: http://lkml.kernel.org/r/20111005184518.GA21601@xxxxxxxxxxxxxxxxxx > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 197088cdb56e..968579b86401 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -244,7 +244,19 @@ void rcu_sched_qs(void) > this_cpu_ptr(&rcu_sched_data), true); > } > > -#ifndef CONFIG_PREEMPT_RT_FULL > +#ifdef CONFIG_PREEMPT_RT_FULL > +static void rcu_preempt_qs(void); > + > +void rcu_bh_qs(void) > +{ > + unsigned long flags; > + > + /* Callers to this function, rcu_preempt_qs(), must disable irqs. */ > + local_irq_save(flags); > + rcu_preempt_qs(); > + local_irq_restore(flags); > +} > +#else > void rcu_bh_qs(void) > { > RCU_LOCKDEP_WARN(preemptible(), "rcu_bh_qs() invoked with preemption enabled!!!"); > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 429a2f144e19..bee9bffeb0ce 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -29,6 +29,7 @@ > #include <linux/oom.h> > #include <linux/sched/debug.h> > #include <linux/smpboot.h> > +#include <linux/jiffies.h> > #include <linux/sched/isolation.h> > #include <uapi/linux/sched/types.h> > #include "../time/tick-internal.h" > @@ -1407,7 +1408,7 @@ static void rcu_prepare_kthreads(int cpu) > > #endif /* #else #ifdef CONFIG_RCU_BOOST */ > > -#if !defined(CONFIG_RCU_FAST_NO_HZ) > +#if !defined(CONFIG_RCU_FAST_NO_HZ) || defined(CONFIG_PREEMPT_RT_FULL) > > /* > * Check to see if any future RCU-related work will need to be done > @@ -1423,7 +1424,9 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt) > *nextevt = KTIME_MAX; > return rcu_cpu_has_callbacks(NULL); > } > +#endif /* !defined(CONFIG_RCU_FAST_NO_HZ) || defined(CONFIG_PREEMPT_RT_FULL) */ > > +#if !defined(CONFIG_RCU_FAST_NO_HZ) > /* > * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up > * after it. > @@ -1520,6 +1523,8 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void) > return cbs_ready; > } > > +#ifndef CONFIG_PREEMPT_RT_FULL > + > /* > * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready > * to invoke. If the CPU has callbacks, try to advance them. Tell the > @@ -1562,6 +1567,7 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt) > *nextevt = basemono + dj * TICK_NSEC; > return 0; > } > +#endif /* #ifndef CONFIG_PREEMPT_RT_FULL */ > > /* > * Prepare a CPU for idle from an RCU perspective. The first major task
