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

[Andrea Parri]

On Tue, Oct 02, 2018 at 02:22:09PM +0100, Will Deacon wrote:
> 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.

I see, thanks for the clarification.

  Andrea


> 
> 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()