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Commit 5fd45516 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Greg Kroah-Hartman
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rcu: Allow for page faults in NMI handlers 

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@linux.vnet.ibm.com



Cc: stable@vger.kernel.org
Fixes: 0be964be ("module: Sanitize RCU usage and locking")
Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 45bd4e40

kernel/rcu/tree.c

+12 −0
Original line number Diff line number Diff line
@@ -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;



	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");

	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!!!");

	oldval = rdtp->dynticks_nesting;

	rdtp->dynticks_nesting++;

	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&