[PATCH v2 3/3] arm64: spinlock: use lock->owner to optimise spin_unlock_wait

[peterz@infradead.org (Peter Zijlstra)]

On Wed, Jun 08, 2016 at 05:25:39PM +0100, Will Deacon wrote:
> Rather than wait until we observe the lock being free, we can also
> return from spin_unlock_wait if we observe that the lock is now held
> by somebody else, which implies that it was unlocked but we just missed
> seeing it in that state.
> 
> Furthermore, in such a scenario there is no longer a need to write back
> the value that we loaded, since we know that there has been a lock
> hand-off, which is sufficient to publish any stores prior to the
> unlock_wait.

You might want a few words on _why_ here. It took me a little while to
figure that out.

Also; human readable arguments to support the thing below go a long way
into validating the test is indeed correct. Because as you've shown,
even the validators cannot be trusted ;-)

> The litmus test is something like:
> 
> AArch64
> {
> 0:X1=x; 0:X3=y;
> 1:X1=y;
> 2:X1=y; 2:X3=x;
> }
>  P0          | P1           | P2           ;
>  MOV W0,#1   | MOV W0,#1    | LDAR W0,[X1] ;
>  STR W0,[X1] | STLR W0,[X1] | LDR W2,[X3]  ;
>  DMB SY      |              |              ;
>  LDR W2,[X3] |              |              ;
> exists
> (0:X2=0 /\ 2:X0=1 /\ 2:X2=0)
> 
> where P0 is doing spin_unlock_wait, P1 is doing spin_unlock and P2 is
> doing spin_lock.

I still have a hard time deciphering these things..