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Date:   Wed, 23 Dec 2020 14:09:37 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...nel.org, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, peterz@...radead.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>
Subject: [PATCH] rcu: Fix dynticks_nmi_nesting underflow check in rcu_is_cpu_rrupt_from_idle

For the smp_call_function() optimization, where callbacks can run from
idle context, in commit 806f04e9fd2c ("rcu: Allow for smp_call_function()
running callbacks from idle"), an additional check is added in
rcu_is_cpu_rrupt_from_idle(), for dynticks_nmi_nesting value being 0,
for these smp_call_function() callbacks running from idle loop.
However, this commit missed updating a preexisting underflow check
of dynticks_nmi_nesting, which checks for a non zero positive value.
Fix this warning and while at it, read the counter only once.

Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
---

Hi,

I was not able to get this warning, with scftorture.

  RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
    "RCU dynticks_nmi_nesting counter underflow/zero!");

Not sure if idle loop smp_call_function() optimization is already present
in mainline?

Another thing, which I am not sure of is, maybe lockdep gets disabled
in the idle loop contexts, where rcu_is_cpu_rrupt_from_idle() is called?
Was this the original intention, to keep the lockdep based
RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0
check separate from idle task context nesting value
WARN_ON_ONCE(!nesting && !is_idle_task(current)) check?


Thanks
Neeraj

 kernel/rcu/tree.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bc8b489..c3037cf 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -457,11 +457,10 @@ static int rcu_is_cpu_rrupt_from_idle(void)
 	/* Check for counter underflows */
 	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
 			 "RCU dynticks_nesting counter underflow!");
-	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
-			 "RCU dynticks_nmi_nesting counter underflow/zero!");
+	nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
+	RCU_LOCKDEP_WARN(nesting < 0, "RCU dynticks_nmi_nesting counter underflow!");
 
 	/* Are we at first interrupt nesting level? */
-	nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
 	if (nesting > 1)
 		return false;
 
-- 
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