On 06/28/2016 07:27 AM, Davidlohr Bueso wrote:
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.
I'll extend the comment: "no locking and no memory barrier"
/*
* 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?
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
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