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Subject[PATCH v2] lib/spinlock_debug.c: prevent an infinite recursive cycle in spin_dump()
Date
It causes an infinite recursive cycle when using CONFIG_DEBUG_SPINLOCK,
in the spin_dump(). Backtrace prints printk() -> console_trylock() ->
do_raw_spin_lock() -> spint_bug() -> spin_dump() -> printk()...
infinitely.

If the spin_bug() is called from a function like printk() which is
trying to obtain the console lock, we should prevent the debug spinlock
code from calling printk() again in that context.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
kernel/locking/spinlock_debug.c | 11 +++++++++++
kernel/printk/printk.c | 5 +++++
2 files changed, 16 insertions(+)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..e745973 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -67,11 +67,22 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
dump_stack();
}

+extern int is_console_lock(raw_spinlock_t *lock);
+
static void spin_bug(raw_spinlock_t *lock, const char *msg)
{
if (!debug_locks_off())
return;

+ /*
+ * If this function is called from a function like printk()
+ * which is trying to obtain the console lock, then we should
+ * not call printk() any more. Or it will cause an infinite
+ * recursive cycle!
+ */
+ if (unlikely(is_console_lock(lock)))
+ return;
+
spin_dump(lock, msg);
}

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826..50ea552 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -119,6 +119,11 @@ static int __down_trylock_console_sem(unsigned long ip)
up(&console_sem);\
} while (0)

+int is_console_lock(raw_spinlock_t *lock)
+{
+ return &console_sem.lock == lock;
+}
+
/*
* This is used for debugging the mess that is the VT code by
* keeping track if we have the console semaphore held. It's
--
1.9.1

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