diff options
| author | Oleg Nesterov | 2014-12-01 22:34:17 +0100 |
|---|---|---|
| committer | Ingo Molnar | 2014-12-08 11:36:44 +0100 |
| commit | 78bff1c8684fb94f1ae7283688f90188b53fc433 (patch) | |
| tree | 48795bcb348947f7e213980570e7413dfc10fc3d | |
| parent | 9fd7fc34cfcaf9f6c932ee1710cce83da3b7bd59 (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.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 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(); + } } /* |