[PATCH] printk: Fix spinlock deadlock in printk reenty

From: linyongting
Date: Wed Nov 30 2016 - 02:17:56 EST

From: Jinling Ke <kejinling@xxxxxxxxxx>

when Oops in printk, printk will call zap_locks() to reinitialize
spinlock to prevent deadlock. In arm, arm64, x86 or other
architecture smp cpu, race condition will occur in printk spinlock
logbuf_lock and then it will result other cpu that is waiting printk
spinlock in deadlock(in function raw_spin_lock). Because the cpus
deadlock, you can see the error printk log:

"SMP: failed to stop secondary CPUs"

In arm, arm64, x86 or other architecture, spinlock variable
is divided into 2 parts, for example they are 'owner' and 'next' in arm.
When get a spinlock, the 'next' part will add 1 and wait 'next' being
equal to 'owner'. However, at this moment, the 'next' part is local
variable, but 'owner' part value is get from global variable logbuf_lock.
However,raw_spin_lock_init(&logbuf_lock) will set 'owner' part and
'next' part to zero, the result is that cpu deadlock in function
raw_spin_lock( while loop in function arch_spin_lock ).

struct of arm spinlock
union {
u32 slock;
struct __raw_tickets {
u16 owner;
u16 next;
} tickets;
} arch_spinlock_t;
static inline void arch_spin_lock(arch_spinlock_t *lock)
<--- At the moment, other cpu call zap_locks()->spin_lock_init(),
<--- set the 'owner' part to zero, but lockval.tickets.next is a
<--- local variable
while (lockval.tickets.next != lockval.tickets.owner) {
lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);

The solution is that In function zap_locks(), replace
raw_spin_lock_init(&logbuf_lock) with raw_spin_unlock(&logbuf_lock),
to let spin_lock stay in unlocked.

Signed-off-by: Jinling Ke <kejinling@xxxxxxxxxx>
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7a55e9..05b1886 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1603,7 +1603,7 @@ static void zap_locks(void)

/* If a crash is occurring, make sure we can't deadlock */
- raw_spin_lock_init(&logbuf_lock);
+ raw_spin_unlock(&logbuf_lock);
/* And make sure that we print immediately */
sema_init(&console_sem, 1);