Messages in this thread | | | Date | Mon, 27 Jun 2016 22:27:17 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race |
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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
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