linux-kernel - [PATCH AUTOSEL 4.9 33/50] kgdb: Fix spurious true from in_dbg

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Date:   Mon,  8 Jun 2020 19:26:23 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Will Deacon <will@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Sasha Levin <sashal@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: [PATCH AUTOSEL 4.9 33/50] kgdb: Fix spurious true from in_dbg_master()

From: Daniel Thompson <daniel.thompson@...aro.org>

[ Upstream commit 3fec4aecb311995189217e64d725cfe84a568de3 ]

Currently there is a small window where a badly timed migration could
cause in_dbg_master() to spuriously return true. Specifically if we
migrate to a new core after reading the processor id and the previous
core takes a breakpoint then we will evaluate true if we read
kgdb_active before we get the IPI to bring us to halt.

Fix this by checking irqs_disabled() first. Interrupts are always
disabled when we are executing the kgdb trap so this is an acceptable
prerequisite. This also allows us to replace raw_smp_processor_id()
with smp_processor_id() since the short circuit logic will prevent
warnings from PREEMPT_DEBUG.

Fixes: dcc7871128e9 ("kgdb: core changes to support kdb")
Suggested-by: Will Deacon <will@...nel.org>
Link: https://lore.kernel.org/r/20200506164223.2875760-1-daniel.thompson@linaro.org
Reviewed-by: Douglas Anderson <dianders@...omium.org>
Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 include/linux/kgdb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index e465bb15912d..6be5545d3584 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -317,7 +317,7 @@ extern void gdbstub_exit(int status);
 extern int			kgdb_single_step;
 extern atomic_t			kgdb_active;
 #define in_dbg_master() \
-	(raw_smp_processor_id() == atomic_read(&kgdb_active))
+	(irqs_disabled() && (smp_processor_id() == atomic_read(&kgdb_active)))
 extern bool dbg_is_early;
 extern void __init dbg_late_init(void);
 #else /* ! CONFIG_KGDB */
-- 
2.25.1