Pardon me for barging in, but I found this whole interchange extremely 
confusing...

On Sat, 8 Jul 2017, Ingo Molnar wrote:

* Paul E. McKenney [off-list ref] wrote:

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On Sat, Jul 08, 2017 at 10:35:43AM +0200, Ingo Molnar wrote:

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* Manfred Spraul [off-list ref] wrote:

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Hi Ingo,

On 07/07/2017 10:31 AM, Ingo Molnar wrote:

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There's another, probably just as significant advantage: queued_spin_unlock_wait()
is 'read-only', while spin_lock()+spin_unlock() dirties the lock cache line. On
any bigger system this should make a very measurable difference - if
spin_unlock_wait() is ever used in a performance critical code path.

At least for ipc/sem:
Dirtying the cacheline (in the slow path) allows to remove a smp_mb() in the
hot path.
So for sem_lock(), I either need a primitive that dirties the cacheline or
sem_lock() must continue to use spin_lock()/spin_unlock().

This statement doesn't seem to make sense.  Did Manfred mean to write 
"smp_mb()" instead of "spin_lock()/spin_unlock()"?

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Technically you could use spin_trylock()+spin_unlock() and avoid the lock acquire 
spinning on spin_unlock() and get very close to the slow path performance of a 
pure cacheline-dirtying behavior.

This is even more confusing.  Did Ingo mean to suggest using 
"spin_trylock()+spin_unlock()" in place of "spin_lock()+spin_unlock()" 
could provide the desired ordering guarantee without delaying other 
CPUs that may try to acquire the lock?  That seems highly questionable.

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But adding something like spin_barrier(), which purely dirties the lock cacheline, 
would be even faster, right?

Interestingly enough, the arm64 and powerpc implementations of
spin_unlock_wait() were very close to what it sounds like you are
describing.

So could we perhaps solve all our problems by defining the generic version thusly:

void spin_unlock_wait(spinlock_t *lock)
{
	if (spin_trylock(lock))
		spin_unlock(lock);
}

How could this possibly be a generic version of spin_unlock_wait()?  
It does nothing at all (with no ordering properties) if some other CPU
currently holds the lock, whereas the real spin_unlock_wait() would
wait until the other CPU released the lock (or possibly longer).

And if no other CPU currently holds the lock, this has exactly the same
performance properties as spin_lock()+spin_unlock(), so what's the
advantage?

Alan Stern

... and perhaps rename it to spin_barrier() [or whatever proper name there would 
be]?

Architectures can still optimize it, to remove the small window where the lock is 
held locally - as long as the ordering is at least as strong as the generic 
version.

This would have various advantages:

 - semantics are well-defined

 - the generic implementation is already pretty well optimized (no spinning)

 - it would make it usable for the IPC performance optimization

 - architectures could still optimize it to eliminate the window where the lock is
   held locally - if there's such instructions available.

Was this proposed before, or am I missing something?

Thanks,

	Ingo