Subject: Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race

From: Davidlohr Bueso <[email-masked]>
Date: Mon, 27 Jun 2016 22:27:17 -0700

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