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.ge2...@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 <rost...@goodmis.org> Reported-by: Joel Fernandes <j...@joelfernandes.org> Reported-by: Andy Lutomirski <l...@kernel.org> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.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
