[PATCH 1/2] x86/spinlocks: Avoid a deadlock when someone unlock a zapped ticked spinlock

[Petr Mladek]

There are few situations when we reinitialize (zap) ticket spinlocks. It
typically happens when the system is going down after an error and we
want to avoid deadlock in some important services. For example,
zap_locks() in printk.c and ioapic_zap_locks().

Peter pointed out that partial deadlock was still possible. It happens
when someone owns a ticket spinlock, we reinitialize it, and the old
owner releases it. Then the head is above the tail and the following
spin_lock() will never[*] succeed.

We could detect this situation in arch_spin_lock() and simply ignore
the superfluous head increment.

We need to do it in the lock() side because the unlock() side works
only with the head to avoid an overflow. Therefore we do not see
the consistent state of the head and the tail there.

Note that we could not check for (head == TICKET_LOCK_INC && !tail)
because the reinitialized lock might be taken several times before
the old owner releases the lock. By other words, the superfluous
head increment might happen at any time.

The change looks quite harmless. It should not affect the fast path
when the lock is taken immediately. It does not make worse the
situation when two processes might own the lock after zapping.
It just avoids the partial deadlock.

[*] unless the ticket number overflows.

Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 arch/x86/include/asm/spinlock.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index be0a05913b91..f732abf57c6f 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -105,12 +105,21 @@ static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
  */
 static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
 {
-	register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
+	register struct __raw_tickets inc;
 
+again:
+	inc = (struct __raw_tickets){ .tail = TICKET_LOCK_INC };
 	inc = xadd(&lock->tickets, inc);
 	if (likely(inc.head == inc.tail))
 		goto out;
 
+	/*
+	 * Avoid a stall when an old owner unlocked a reinitialized spinlock.
+	 * Simply ignore the superfluous increment of the head.
+	 */
+	if (unlikely(inc.head == inc.tail + TICKET_LOCK_INC))
+		goto again;
+
 	for (;;) {
 		unsigned count = SPIN_THRESHOLD;
 
-- 
1.8.5.6

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