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Date:   Tue, 2 Oct 2018 14:31:52 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Will Deacon <will.deacon@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org,
        linux-kernel@vger.kernel.org, longman@redhat.com,
        tglx@linutronix.de
Subject: Re: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86
Message-ID: <20181002123152.GA10055@andrea>
References: <20180926110117.405325143@infradead.org>
 <20180926111307.513429499@infradead.org>
 <20181001171700.GC13918@arm.com>
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> consider this scenario with your patch:
> 
> 1. CPU0 sees a locked val, and is about to do your xchg_relaxed() to set
>    pending.
> 
> 2. CPU1 comes in and sets pending, spins on locked
> 
> 3. CPU2 sees a pending and locked val, and is about to enter the head of
>    the waitqueue (i.e. it's right before xchg_tail()).
> 
> 4. The locked holder unlock()s, CPU1 takes the lock() and then unlock()s
>    it, so pending and locked are now 0.
> 
> 5. CPU0 sets pending and reads back zeroes for the other fields
> 
> 6. CPU0 clears pending and sets locked -- it now has the lock
> 
> 7. CPU2 updates tail, sees it's at the head of the waitqueue and spins
>    for locked and pending to go clear. However, it reads a stale value
>    from step (4) and attempts the atomic_try_cmpxchg() to take the lock.
> 
> 8. CPU2 will fail the cmpxchg(), but then go ahead and set locked. At this
>    point we're hosed, because both CPU2 and CPU0 have the lock.

Thanks for pointing this out.   I am wondering: can't we have a similar
scenario with the current code (i.e., w/o these patches): what prevents
the scenario reported below, following Peter's diagram, from happening?

  Andrea


  CPU0		CPU1		CPU2		CPU3

0)						lock
						  trylock -> (0,0,1)
1)lock
    trylock /* fail */

2)		lock
		  trylock /* fail */
		  fetch_or_acquire -> (0,1,1)
		  wait-locked

3)				lock
				  trylock /* fail */
				  goto queue

4)						unlock -> (0,1,0)
		  clr_pnd_set_lck -> (0,0,1)
		  unlock -> (0,0,0)

5)  fetch_or_acquire -> (0,1,0)
6)  clr_pnd_set_lck -> (0,0,1)
7)				  xchg_tail -> (n,0,1)
				  load_acquire <- (n,0,0) (from-4)
8)				  cmpxchg /* fail */
				  set_locked()