Re: [PATCH v2 0/9] Remove spin_unlock_wait()

[Alan Stern]

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

On Sat, 8 Jul 2017, Ingo Molnar wrote:

> * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Sat, Jul 08, 2017 at 10:35:43AM +0200, Ingo Molnar wrote:
> > > 
> > > * Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
> > > 
> > > > Hi Ingo,
> > > > 
> > > > On 07/07/2017 10:31 AM, Ingo Molnar wrote:
> > > > > 
> > > > > 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()"?

> > > 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.

> > > 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