linux-kernel - [PATCH] kgdb: Fix spurious true from in_dbg

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Message-Id: <20200506164223.2875760-1-daniel.thompson@linaro.org>
Date:   Wed,  6 May 2020 17:42:23 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        patches@...aro.org, Will Deacon <will@...nel.org>
Subject: [PATCH] kgdb: Fix spurious true from in_dbg_master()

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>
Signed-off-by: Daniel Thompson <daniel.thompson@...aro.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 b072aeb1fd78..4d6fe87fd38f 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -323,7 +323,7 @@ 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);
 extern void kgdb_panic(const char *msg);

base-commit: 6a8b55ed4056ea5559ebe4f6a4b247f627870d4c
--
2.25.1