Hi Thomas, On Tue, May 05, 2020 at 03:44:05PM +0200, Thomas Gleixner wrote:
Thank you for CC'ing me. > Interrupts and exceptions invoke rcu_irq_enter() on entry and need to > invoke rcu_irq_exit() before they either return to the interrupted code or > invoke the scheduler due to preemption. > > The general assumption is that RCU idle code has to have preemption > disabled so that a return from interrupt cannot schedule. So the return > from interrupt code invokes rcu_irq_exit() and preempt_schedule_irq(). > > If there is any imbalance in the rcu_irq/nmi* invocations or RCU idle code > had preemption enabled then this goes unnoticed until the CPU goes idle or > some other RCU check is executed. > > Provide rcu_irq_exit_preempt() which can be invoked from the > interrupt/exception return code in case that preemption is enabled. It > invokes rcu_irq_exit() and contains a few sanity checks in case that > CONFIG_PROVE_RCU is enabled to catch such issues directly. Could you let me know which patch or part in the multi-part series is using it? > > Signed-off-by: Thomas Gleixner <t...@linutronix.de> > Cc: "Paul E. McKenney" <paul...@kernel.org> > Cc: Joel Fernandes <j...@joelfernandes.org> > --- > include/linux/rcutiny.h | 1 + > include/linux/rcutree.h | 1 + > kernel/rcu/tree.c | 21 +++++++++++++++++++++ > 3 files changed, 23 insertions(+) > > --- a/include/linux/rcutiny.h > +++ b/include/linux/rcutiny.h > @@ -71,6 +71,7 @@ static inline void rcu_irq_enter(void) { > static inline void rcu_irq_exit_irqson(void) { } > static inline void rcu_irq_enter_irqson(void) { } > static inline void rcu_irq_exit(void) { } > +static inline void rcu_irq_exit_preempt(void) { } > static inline void exit_rcu(void) { } > static inline bool rcu_preempt_need_deferred_qs(struct task_struct *t) > { > --- a/include/linux/rcutree.h > +++ b/include/linux/rcutree.h > @@ -46,6 +46,7 @@ void rcu_idle_enter(void); > void rcu_idle_exit(void); > void rcu_irq_enter(void); > void rcu_irq_exit(void); > +void rcu_irq_exit_preempt(void); > void rcu_irq_enter_irqson(void); > void rcu_irq_exit_irqson(void); > > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -706,6 +706,27 @@ void noinstr rcu_irq_exit(void) > rcu_nmi_exit(); > } > > +/** > + * rcu_irq_exit_preempt - Inform RCU that current CPU is exiting irq > + * towards in kernel preemption > + * > + * Same as rcu_irq_exit() but has a sanity check that scheduling is safe > + * from RCU point of view. Invoked from return from interrupt before kernel > + * preemption. > + */ > +void rcu_irq_exit_preempt(void) > +{ > + lockdep_assert_irqs_disabled(); > + rcu_nmi_exit(); > + > + RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) <= 0, > + "RCU dynticks_nesting counter underflow/zero!"); Makes sense. > + RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0, > + "RCU dynticks_nmi_nesting counter underflow/zero!"); This new function will be called only from the outer-most IRQ that interrupted kernel mode (process context). Right? If so, a better (more specific) check for the second RCU_LOCKDEP_WARN above is: RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) != DYNTICK_IRQ_NONIDLE, "Bad RCU dynticks_nmi_nesting counter\n"); That will make sure, it is only called from outer-most rcu_irq_exit() and interrupting kernel mode. Or, if [1] is merged, then we could just combine the checks into one check. RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) != 1, "Bad RCU dynticks_nmi_nesting counter\n"); > + RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(), > + "RCU in extended quiescent state!"); Makes sense. BTW, I wonder if a better place to do this "don't enter scheduler while RCU is not watching" is rcu_note_context_switch()... thanks, - Joel [1] https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=rcu-dynticks-may4-rebased&id=b48863c234295d8ec956b50f6cf5ae0a0269f48d > +} > + > /* > * Wrapper for rcu_irq_exit() where interrupts are enabled. > * >