[PATCH 1/1] x86, ticketlock: spin_unlock_wait() can livelock

[Oleg Nesterov]

arch_spin_unlock_wait() looks very suboptimal, to the point I think
this is just wrong and can lead to livelock: if the lock is heavily
contended we can never see head == tail.

But we do not need to wait for arch_spin_is_locked() == F. If it is
locked we only need to wait until the current owner drops this lock.
So we could simply spin until old_head != lock->tickets.head in this
case, but .head can overflow and thus we can't check "unlocked" only
once before the main loop.

Also, the "unlocked" check can ignore TICKET_SLOWPATH_FLAG bit.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 9295016..a4efe47 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -183,8 +183,20 @@ static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
 
 static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
 {
-	while (arch_spin_is_locked(lock))
+	__ticket_t head = ACCESS_ONCE(lock->tickets.head);
+
+	for (;;) {
+		struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
+		/*
+		 * We need to check "unlocked" in a loop, tmp.head == head
+		 * can be false positive because of overflow.
+		 */
+		if (tmp.head == (tmp.tail & ~TICKET_SLOWPATH_FLAG) ||
+		    tmp.head != head)
+			break;
+
 		cpu_relax();
+	}
 }
 
 /*
-- 
1.5.5.1


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