linux-kernel - Re: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86

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Date:   Tue, 2 Oct 2018 14:22:09 +0100
From:   Will Deacon <will.deacon@....com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, longman@...hat.com,
        tglx@...utronix.de
Subject: Re: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86

On Tue, Oct 02, 2018 at 02:31:52PM +0200, Andrea Parri wrote:
> > 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?

The xchg_tail() in step (7) reads from the fetch_or_acquire() in step (5),
so I don't think we can see a stale value in the subsequent (overlapping)
acquire load.

Will

>   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()