From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org (Peter Zijlstra)
Date: Fri, 10 Jun 2016 14:25:20 +0200
Subject: [PATCH v2 3/3] arm64: spinlock: use lock->owner to optimise
 spin_unlock_wait
In-Reply-To: <1465403139-21054-3-git-send-email-will.deacon@arm.com>
References: <1465403139-21054-1-git-send-email-will.deacon@arm.com>
 <1465403139-21054-3-git-send-email-will.deacon@arm.com>
Message-ID: <20160610122520.GC30154@twins.programming.kicks-ass.net>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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..