On 08/12/2016 10:01 AM, Waiman Long wrote:
> The reason for using a special lock is that I want both sequence number
> update and locking to be done together atomically. They can be made
> separate as is in the seqlock. However, that will make the code more
> complex to make sure that all the threads see a consistent set of lock
> state and sequence number.
Why do we need a sequence number? The "cached" HPET itself could be used.
I'm thinking something like below could use a spinlock instead of the
doing a custom cmpxchg sequence. The spin_is_locked() should allow the
contended "readers" to avoid using atomics.
spinlock_t hpet_lock;
u32 hpet_value;
...
{
u32 old_hpet = READ_ONCE(hpet_value);
u32 new_hpet;
// need to ensure that the spin_is_locked() is ordered after
// the READ_ONCE().
smp_rmb();
// spin_is_locked() doesn't do atomics
if (!spin_is_locked(&hpet_lock) && spin_trylock(&hpet_lock)) {
WRITE_ONCE(hpet_value, real_read_hpet());
spin_unlock(&hpet_lock);
return hpet_value;
}
// Contended case. We spin here waiting for the guy who holds
// the lock to write a new value to 'hpet_value'.
//
// We know that our old_hpet is older than our check for the
// spinlock being locked. So, someone must either have already
// updated it or be updating it.
do {
cpu_relax();
// We do not do a rmb() here. We don't need a guarantee
// that this read is up-to-date, just that it will
// _eventually_ see an up-to-date value.
new_hpet = READ_ONCE(hpet_value);
} while (old_hpet == new_hpet);
return new_hpet;
}
