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authorOleg Nesterov <oleg@redhat.com>2014-12-01 22:34:17 +0100
committerIngo Molnar <mingo@kernel.org>2014-12-08 11:36:44 +0100
commit78bff1c8684fb94f1ae7283688f90188b53fc433 (patch)
tree48795bcb348947f7e213980570e7413dfc10fc3d
parent9fd7fc34cfcaf9f6c932ee1710cce83da3b7bd59 (diff)
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@redhat.com> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Jeremy Fitzhardinge <jeremy@goop.org> Cc: Paul E.McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Waiman Long <Waiman.Long@hp.com> Link: http://lkml.kernel.org/r/20141201213417.GA5842@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--arch/x86/include/asm/spinlock.h14
1 files changed, 13 insertions, 1 deletions
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index bf156ded74b5..abc34e95398d 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();
+ }
}
/*