On Mon, 2017-10-16 at 18:11 +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote: > 4.4-stable review patch. If anyone has any objections, please let me know. > > ------------------ > > From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > commit 28585a832602747cbfa88ad8934013177a3aae38 upstream. > > A number of architecture invoke rcu_irq_enter() on exception entry in > order to allow RCU read-side critical sections in the exception handler > when the exception is from an idle or nohz_full CPU. This works, at > least unless the exception happens in an NMI handler. In that case, > rcu_nmi_enter() would already have exited the extended quiescent state, > which would mean that rcu_irq_enter() would (incorrectly) cause RCU > to think that it is again in an extended quiescent state. This will > in turn result in lockdep splats in response to later RCU read-side > critical sections. > > This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to > take no action if there is an rcu_nmi_enter() in effect, thus avoiding > the unscheduled return to RCU quiescent state. This in turn should > make the kernel safe for on-demand RCU voyeurism. > > Link: http://lkml.kernel.org/r/20170922211022.GA18084@xxxxxxxxxxxxxxxxxx > > Cc: stable@xxxxxxxxxxxxxxx > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking") > > Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > --- > kernel/rcu/tree.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -759,6 +759,12 @@ void rcu_irq_exit(void) > > local_irq_save(flags); > rdtp = this_cpu_ptr(&rcu_dynticks); > + > + /* Page faults can happen in NMI handlers, so check... */ > + if (READ_ONCE(rdtp->dynticks_nmi_nesting)) > + return; Shouldn't there be a local_irq_restore() on this return path? Or does this condition imply that IRQs were already disabled? > + RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!"); I don't see why you added RCU_LOCKDEP_WARN() here. Prior to 4.5 it's not an error to call this function with IRQs disabled. And after calling local_irq_save(), it's redundant to assert that IRQs are disabled. > oldval = rdtp->dynticks_nesting; > rdtp->dynticks_nesting--; > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && > @@ -887,6 +893,12 @@ void rcu_irq_enter(void) > > local_irq_save(flags); > rdtp = this_cpu_ptr(&rcu_dynticks); > + > + /* Page faults can happen in NMI handlers, so check... */ > + if (READ_ONCE(rdtp->dynticks_nmi_nesting)) > + return; > + > + RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!"); Same problems here. Ben. > oldval = rdtp->dynticks_nesting; > rdtp->dynticks_nesting++; > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && -- Ben Hutchings Software Developer, Codethink Ltd.