Commit-ID: e11ec65cc8d63c41fc468363b65826a5ae4b8c66
Gitweb: https://git.kernel.org/tip/e11ec65cc8d63c41fc468363b65826a5ae4b8c66
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Thu, 28 Jun 2018 12:45:23 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Thu, 30 Aug 2018 16:02:36 -0700
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@xxxxxxxxxxxxxxxxxx
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@xxxxxxxxxxx>
Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 346624716d6e..0b42249e2e40 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);
@@ -896,6 +897,7 @@ 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);
}
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