On 06/28/2016 07:27 AM, Davidlohr Bueso wrote:

On Thu, 23 Jun 2016, Manfred Spraul wrote:

quoted

What I'm not sure yet is if smp_load_acquire() is sufficient:

Thread A:

quoted

      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.

I'll extend the comment: "no locking and no memory barrier"

quoted

quoted

               /*
                * 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:

quoted

      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?

You are right, I didn't read this patch fully.
If I understand it right, it means that spin_lock() is both an acquire 
and a release - for qspinlocks.

It this valid for all spinlock implementations, for all architectures?
Otherwise: How can I detect in generic code if I can rely on a release 
inside spin_lock()?

--
     Manfred