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

[Davidlohr Bueso]

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