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]> --- 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 -- 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/
