Subject: rcu: Make ksoftirqd do RCU quiescent states From: "Paul E. McKenney" Date: Wed, 5 Oct 2011 11:45:18 -0700 Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.1/patches-4.1.3-rt3.tar.xz 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. Signed-off-by: Paul E. McKenney Link: http://lkml.kernel.org/r/20111005184518.GA21601@linux.vnet.ibm.com Signed-off-by: Thomas Gleixner --- include/linux/rcupdate.h | 6 ------ kernel/rcu/tree.c | 9 ++++++++- kernel/rcu/tree_plugin.h | 9 +++++++-- 3 files changed, 15 insertions(+), 9 deletions(-) --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -300,13 +300,7 @@ static inline int rcu_preempt_depth(void void rcu_init(void); void rcu_end_inkernel_boot(void); void rcu_sched_qs(void); - -#ifdef CONFIG_PREEMPT_RT_FULL -static inline void rcu_bh_qs(void) { } -#else void rcu_bh_qs(void); -#endif - void rcu_check_callbacks(int user); struct notifier_block; void rcu_idle_enter(void); --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -220,7 +220,14 @@ void rcu_sched_qs(void) } } -#ifndef CONFIG_PREEMPT_RT_FULL +#ifdef CONFIG_PREEMPT_RT_FULL +static void rcu_preempt_qs(void); + +void rcu_bh_qs(void) +{ + rcu_preempt_qs(); +} +#else void rcu_bh_qs(void) { if (!__this_cpu_read(rcu_bh_data.passed_quiesce)) { --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -28,6 +28,7 @@ #include #include #include +#include #include "../time/tick-internal.h" #ifdef CONFIG_RCU_BOOST @@ -1356,7 +1357,7 @@ 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 @@ -1374,7 +1375,9 @@ int rcu_needs_cpu(unsigned long *delta_j return rcu_cpu_has_callbacks(NULL); } #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */ +#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. @@ -1472,6 +1475,8 @@ static bool __maybe_unused rcu_try_advan 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 @@ -1512,7 +1517,7 @@ int rcu_needs_cpu(unsigned long *dj) return 0; } #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */ - +#endif /* #ifndef CONFIG_PREEMPT_RT_FULL */ /* * Prepare a CPU for idle from an RCU perspective. The first major task * is to sense whether nohz mode has been enabled or disabled via sysfs.