Re: [PATCH 3/3] spinlock_debug: panic on recursive lock spin_dump()

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (01/31/16 21:33), Sergey Senozhatsky wrote:
> spin_dump() calls printk() which can attempt to reacquire the
> 'buggy' lock (one of printk's lock, or console device driver lock,
> etc.) and thus spin_dump() will recurse into itself.

how about splitting ->owner_cpu 4 bytes as:

                            |                                   |
  1 byte spin bug recursion | 1 byte spin_dump recursion counter | 2 bytes owner cpu
                            |                                   |

?

so we have 2 bytes to store the lock owner's smp_processor_id().
(I just don't want to extend spin lock with another 4 bytes to
fix the recursion.)


spin_dump() will increment lock's spin_dump recursion counter and decrement it
upon the exit from spin_dump(). if lock's recursion counter is equal to
U8_MAX -- panic() the system, we have recursed spin_dump() into spin_dump()
enough... (U8_MAX recursive printk->spin_dump->printk calls look big enough).

for spin_bug() recursion we don't have to maintain any counter: set
SPIN_BUG_RECURSION bit at the beginning of spin_bug() and clear it at
the end. if the lock already has SPIN_BUG_RECURSION bit set -- panic()
the system.

compile tested only.
will test it tomorrow.

---
 kernel/locking/spinlock_debug.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..54bcc3b 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -13,6 +13,9 @@
 #include <linux/delay.h>
 #include <linux/export.h>
 
+#define SPIN_DUMP_RECURSION	(1 << 16)
+#define SPIN_BUG_RECURSION	(1 << 31)
+
 void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name,
 			  struct lock_class_key *key)
 {
@@ -26,7 +29,7 @@ void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name,
 	lock->raw_lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
 	lock->magic = SPINLOCK_MAGIC;
 	lock->owner = SPINLOCK_OWNER_INIT;
-	lock->owner_cpu = -1;
+	lock->owner_cpu = USHRT_MAX;
 }
 
 EXPORT_SYMBOL(__raw_spin_lock_init);
@@ -44,14 +47,26 @@ void __rwlock_init(rwlock_t *lock, const char *name,
 	lock->raw_lock = (arch_rwlock_t) __ARCH_RW_LOCK_UNLOCKED;
 	lock->magic = RWLOCK_MAGIC;
 	lock->owner = SPINLOCK_OWNER_INIT;
-	lock->owner_cpu = -1;
+	lock->owner_cpu = USHRT_MAX;
 }
 
 EXPORT_SYMBOL(__rwlock_init);
 
+static void spin_recursion_panic(raw_spinlock_t *lock, const char *msg)
+{
+	panic("BUG: lock: %pS %s recursion on CPU#%d, %s/%d\n",
+			lock, msg, raw_smp_processor_id(),
+			current->comm, task_pid_nr(current));
+}
+
 static void spin_dump(raw_spinlock_t *lock, const char *msg)
 {
 	struct task_struct *owner = NULL;
+	u8 recursion_counter = (u8)(lock->owner_cpu >> 16);
+
+	lock->owner_cpu += SPIN_DUMP_RECURSION;
+	if ((recursion_counter + 1) == U8_MAX)
+		spin_recursion_panic(lock, "spin_dump()");
 
 	if (lock->owner && lock->owner != SPINLOCK_OWNER_INIT)
 		owner = lock->owner;
@@ -63,8 +78,10 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
 		lock, lock->magic,
 		owner ? owner->comm : "<none>",
 		owner ? task_pid_nr(owner) : -1,
-		lock->owner_cpu);
+		lock->owner_cpu == USHRT_MAX ? -1 : lock->owner_cpu);
 	dump_stack();
+
+	lock->owner_cpu -= SPIN_DUMP_RECURSION;
 }
 
 static void spin_bug(raw_spinlock_t *lock, const char *msg)
@@ -72,7 +89,12 @@ static void spin_bug(raw_spinlock_t *lock, const char *msg)
 	if (!debug_locks_off())
 		return;
 
+	if (lock->owner_cpu & SPIN_BUG_RECURSION)
+		spin_recursion_panic(lock, "spin_bug()");
+
+	lock->owner_cpu |= SPIN_BUG_RECURSION;
 	spin_dump(lock, msg);
+	lock->owner_cpu &= ~SPIN_BUG_RECURSION;
 }
 
 #define SPIN_BUG_ON(cond, lock, msg) if (unlikely(cond)) spin_bug(lock, msg)
@@ -100,7 +122,7 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
 	SPIN_BUG_ON(lock->owner_cpu != raw_smp_processor_id(),
 							lock, "wrong CPU");
 	lock->owner = SPINLOCK_OWNER_INIT;
-	lock->owner_cpu = -1;
+	lock->owner_cpu = USHRT_MAX;
 }
 
 static void __spin_lock_debug(raw_spinlock_t *lock)
@@ -244,7 +266,7 @@ static inline void debug_write_unlock(rwlock_t *lock)
 	RWLOCK_BUG_ON(lock->owner_cpu != raw_smp_processor_id(),
 							lock, "wrong CPU");
 	lock->owner = SPINLOCK_OWNER_INIT;
-	lock->owner_cpu = -1;
+	lock->owner_cpu = USHRT_MAX;
 }
 
 #if 0		/* This can cause lockups */
-- 
2.7.0.75.g3ee1e0f