linux-kernel - [PATCH -v5 6/5] context_tracking: fix exception_enter when already in IN

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Date:	Wed, 11 Feb 2015 14:43:19 -0500
From:	Rik van Riel <riel@...hat.com>
To:	fweisbec@...il.com
Cc:	paulmck@...ux.vnet.ibm.com, luto@...capital.net,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, mtosatti@...hat.com, borntraeger@...ibm.com,
	lcapitulino@...hat.com, pbonzini@...hat.com
Subject: [PATCH -v5 6/5] context_tracking: fix exception_enter when already
 in IN_KERNEL

If exception_enter happens when already in IN_KERNEL state, the
code still calls context_tracking_exit, which ends up in
rcu_eqs_exit_common, which explodes with a WARN_ON when it is
called in a situation where dynticks are not enabled.

This can be avoided by having exception_enter only switch to
IN_KERNEL state if the current state is not already IN_KERNEL.

Signed-off-by: Rik van Riel <riel@...hat.com>
Reported-by: Luiz Capitulino <lcapitulino@...hat.com>
---
Frederic, you will want this "bonus" patch, too :)

Thanks to Luiz for finding this one. Whatever I was running did not
trigger this issue...

 include/linux/context_tracking.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index b65fd1420e53..9da230406e8c 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -37,7 +37,8 @@ static inline enum ctx_state exception_enter(void)
 		return 0;
 
 	prev_ctx = this_cpu_read(context_tracking.state);
-	context_tracking_exit(prev_ctx);
+	if (prev_ctx != IN_KERNEL)
+		context_tracking_exit(prev_ctx);
 
 	return prev_ctx;
 }

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