[RFC v2 01/10] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb

From: Daniel Thompson
Date: Wed Apr 02 2014 - 11:44:31 EST

If kdb is triggered using SysRq-g then any use of the sr command results
in the SysRq key table lock being recursively acquired, killing the debug
session. That patch resolves the problem by introducing a _nolock
alternative for __handle_sysrq.

Strictly speaking this approach risks racing on the key table when kdb is
triggered by something other than SysRq-g however in that case any other
CPU involved should release the spin lock before kgdb parks the slave

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
drivers/tty/sysrq.c | 11 ++++++++---
include/linux/sysrq.h | 1 +
kernel/debug/kdb/kdb_main.c | 2 +-
3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index ce396ec..7b47b2d 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -505,14 +505,12 @@ static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
sysrq_key_table[i] = op_p;

-void __handle_sysrq(int key, bool check_mask)
+void __handle_sysrq_nolock(int key, bool check_mask)
struct sysrq_key_op *op_p;
int orig_log_level;
int i;
- unsigned long flags;

- spin_lock_irqsave(&sysrq_key_table_lock, flags);
* Raise the apparent loglevel to maximum so that the sysrq header
* is shown to provide the user with positive feedback. We do not
@@ -554,6 +552,13 @@ void __handle_sysrq(int key, bool check_mask)
console_loglevel = orig_log_level;
+void __handle_sysrq(int key, bool check_mask)
+ unsigned long flags;
+ spin_lock_irqsave(&sysrq_key_table_lock, flags);
+ __handle_sysrq_nolock(key, check_mask);
spin_unlock_irqrestore(&sysrq_key_table_lock, flags);

diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
index 387fa7d..1d51d64 100644
--- a/include/linux/sysrq.h
+++ b/include/linux/sysrq.h
@@ -44,6 +44,7 @@ struct sysrq_key_op {

void handle_sysrq(int key);
void __handle_sysrq(int key, bool check_mask);
+void __handle_sysrq_nolock(int key, bool check_mask);
int register_sysrq_key(int key, struct sysrq_key_op *op);
int unregister_sysrq_key(int key, struct sysrq_key_op *op);
struct sysrq_key_op *__sysrq_get_key_op(int key);
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 0b097c8..f39f926 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -1924,7 +1924,7 @@ static int kdb_sr(int argc, const char **argv)
if (argc != 1)
- __handle_sysrq(*argv[1], false);
+ __handle_sysrq_nolock(*argv[1], false);

return 0;

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