[tip:core/locking] x86/ticketlock: Fix spin_unlock_wait() livelock

[tip-bot for Oleg Nesterov]

Commit-ID:  78bff1c8684fb94f1ae7283688f90188b53fc433
Gitweb:     http://git.kernel.org/tip/78bff1c8684fb94f1ae7283688f90188b53fc433
Author:     Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate: Mon, 1 Dec 2014 22:34:17 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 8 Dec 2014 11:36:44 +0100

x86/ticketlock: Fix spin_unlock_wait() livelock

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>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Cc: Paul E.McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Waiman Long <Waiman.Long@xxxxxx>
Link: http://lkml.kernel.org/r/20141201213417.GA5842@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index bf156de..abc34e9 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -184,8 +184,20 @@ 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();
+	}
 }
 
 /*
--
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