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Message-Id: <1457692627-14076-3-git-send-email-byungchul.park@lge.com>
Date: Fri, 11 Mar 2016 19:37:07 +0900
From: Byungchul Park <byungchul.park@....com>
To: akpm@...ux-foundation.org, mingo@...nel.org
Cc: linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
sergey.senozhatsky.work@...il.com, peter@...leysoftware.com,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org
Subject: [PATCH v6 2/2] printk: Make printing of spin_dump() deferred to avoid a deadlock
In the case that console_sem.lock has already been held, printk() must
not be performed in normal way, otherwise a deadlock can happen because
it tries to acquire the console_sem.lock again, e.i. deadlock. This
patch defers the actual printing of printk(), using irq_work_queue().
The scenario the bad thing can happen is,
printk
console_trylock
console_unlock
up_console_sem
up
raw_spin_lock_irqsave(&sem->lock, flags)
__up
wake_up_process
try_to_wake_up
raw_spin_lock_irqsave(&p->pi_lock)
__spin_lock_debug
spin_dump
printk
console_trylock
raw_spin_lock_irqsave(&sem->lock, flags)
>>> DEADLOCK <<<
Signed-off-by: Byungchul Park <byungchul.park@....com>
---
kernel/locking/spinlock_debug.c | 33 ++++++++++++++++++++++++++++++++-
kernel/printk/printk.c | 8 ++++++++
2 files changed, 40 insertions(+), 1 deletion(-)
diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..fd24588 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -49,7 +49,7 @@ void __rwlock_init(rwlock_t *lock, const char *name,
EXPORT_SYMBOL(__rwlock_init);
-static void spin_dump(raw_spinlock_t *lock, const char *msg)
+static void __spin_dump(raw_spinlock_t *lock, const char *msg)
{
struct task_struct *owner = NULL;
@@ -67,6 +67,37 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
dump_stack();
}
+extern int console_sem_spin_is_held(void);
+
+static void __spin_dump_deferred(raw_spinlock_t *lock, const char *msg)
+{
+ printk_func_t s;
+
+ s = this_cpu_read(printk_func);
+ this_cpu_write(printk_func, vprintk_deferred);
+
+ /*
+ * To change printk_func, it must be in preempt disabled and irq
+ * disabled. WARN_ON() should be called after the change because
+ * the default printk_func which may be called from WARN_ON()
+ * is prohibited in this context.
+ */
+ WARN_ON(!preempt_count() || !irqs_disabled());
+ __spin_dump(lock, msg);
+
+ this_cpu_write(printk_func, s);
+
+ printk_pending_output();
+}
+
+static void spin_dump(raw_spinlock_t *lock, const char *msg)
+{
+ if (unlikely(console_sem_spin_is_held()))
+ __spin_dump_deferred(lock, msg);
+ else
+ __spin_dump(lock, msg);
+}
+
static void spin_bug(raw_spinlock_t *lock, const char *msg)
{
if (!debug_locks_off())
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e82fae4..2e6b008 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2188,6 +2188,14 @@ int is_console_locked(void)
return console_locked;
}
+#ifdef CONFIG_DEBUG_SPINLOCK
+int console_sem_spin_is_held(void)
+{
+ return raw_spin_is_locked(&console_sem.lock) &&
+ (console_sem.lock.owner == current);
+}
+#endif
+
static void console_cont_flush(char *text, size_t size)
{
unsigned long flags;
--
1.9.1
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