rcu: Allow for page faults in NMI handlers

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@linux.vnet.ibm.com
Cc: stable@vger.kernel.org
Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51d4c3a..63bee8e 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -888,6 +888,11 @@ void rcu_irq_exit(void)
 
        RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
        rdtp = this_cpu_ptr(&rcu_dynticks);
+
+       /* Page faults can happen in NMI handlers, so check... */
+       if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+               return;
+
        WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
                     rdtp->dynticks_nesting < 1);
        if (rdtp->dynticks_nesting <= 1) {
@@ -1020,6 +1025,11 @@ void rcu_irq_enter(void)
 
        RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
        rdtp = this_cpu_ptr(&rcu_dynticks);
+
+       /* Page faults can happen in NMI handlers, so check... */
+       if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+               return;
+
        oldval = rdtp->dynticks_nesting;
        rdtp->dynticks_nesting++;
        WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&