On Thu, 23 Jun 2016, Manfred Spraul wrote:
What I'm not sure yet is if smp_load_acquire() is sufficient: Thread A:if (!READ_ONCE(sma->complex_mode)) {The code is test_and_test, no barrier requirements for first test
Yeah, it would just make us take the big lock unnecessarily, nothing fatal and I agree its probably worth the optimization. It still might be worth commenting.
/* * It appears that no complex operation is around. * Acquire the per-semaphore lock. */ spin_lock(&sem->lock); if (!smp_load_acquire(&sma->complex_mode)) { /* fast path successful! */ return sops->sem_num; } spin_unlock(&sem->lock); }Thread B:WRITE_ONCE(sma->complex_mode, true); /* We need a full barrier: * The write to complex_mode must be visible * before we read the first sem->lock spinlock state. */ smp_mb(); for (i = 0; i < sma->sem_nsems; i++) { sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); }If thread A is allowed to issue read_spinlock;read complex_mode;write spinlock, then thread B would not notice that thread A is in the critical section
Are you referring to the sem->lock word not being visibly locked before we read complex_mode (for the second time)? This issue was fixed in 2c610022711 (locking/qspinlock: Fix spin_unlock_wait() some more). So smp_load_acquire should be enough afaict, or are you referring to something else? Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html