Commit-ID: 78bff1c8684fb94f1ae7283688f90188b53fc433 Gitweb: http://git.kernel.org/tip/78bff1c8684fb94f1ae7283688f90188b53fc433 Author: Oleg Nesterov <[email protected]> AuthorDate: Mon, 1 Dec 2014 22:34:17 +0100 Committer: Ingo Molnar <[email protected]> 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 <[email protected]> Acked-by: Linus Torvalds <[email protected]> Cc: Jeremy Fitzhardinge <[email protected]> Cc: Paul E.McKenney <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Waiman Long <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- 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(); + } } /* -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
