linux-kernel - [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts

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Message-Id: <20180829222047.319-6-paulmck@linux.vnet.ibm.com>
Date:   Wed, 29 Aug 2018 15:20:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts

RCU's dyntick-idle code is written to tolerate half-interrupts, that it,
either an interrupt that invokes rcu_irq_enter() but never invokes the
corresponding rcu_irq_exit() on the one hand, or an interrupt that never
invokes rcu_irq_enter() but does invoke the "corresponding" rcu_irq_exit()
on the other.  These things really did happen at one time, as evidenced
by this ca-2011 LKML post:

http://lkml.kernel.org/r/20111014170019.GE2428@linux.vnet.ibm.com

The reason why RCU tolerates half-interrupts is that usermode helpers
used exceptions to invoke a system call from within the kernel such that
the system call did a normal return (not a return from exception) to
the calling context.  This caused rcu_irq_enter() to be invoked without
a matching rcu_irq_exit().  However, usermode helpers have since been
rewritten to make much more housebroken use of workqueues, kernel threads,
and do_execve(), and therefore should no longer produce half-interrupts.
No one knows of any other source of half-interrupts, but then again,
no one seems insane enough to go audit the entire kernel to verify that
half-interrupts really are a relic of the past.

This commit therefore adds a pair of WARN_ON_ONCE() calls that will
trigger in the presence of half interrupts, which the code will continue
to handle correctly.  If neither of these WARN_ON_ONCE() trigger by
mid-2021, then perhaps RCU can stop handling half-interrupts, which
would be a considerable simplification.

Reported-by: Steven Rostedt <rostedt@...dmis.org>
Reported-by: Joel Fernandes <joel@...lfernandes.org>
Reported-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dc041c2afbcc..d2b6ade692c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -714,6 +714,7 @@ static void rcu_eqs_enter(bool user)
 	struct rcu_dynticks *rdtp;

 	rdtp = this_cpu_ptr(&rcu_dynticks);
+	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
 	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0);
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 		     rdtp->dynticks_nesting == 0);
@@ -895,6 +896,7 @@ static void rcu_eqs_exit(bool user)
 	trace_rcu_dyntick(TPS("End"), rdtp->dynticks_nesting, 1, rdtp->dynticks);
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
 	WRITE_ONCE(rdtp->dynticks_nesting, 1);
+	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting);
 	WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
 }

-- 
2.17.1