Re: spin_lock implicit/explicit memory barrier

[Davidlohr Bueso]

On Wed, 10 Aug 2016, Manfred Spraul wrote:

> On 08/10/2016 02:05 AM, Benjamin Herrenschmidt wrote:
> > On Tue, 2016-08-09 at 20:52 +0200, Manfred Spraul wrote:
> > > Hi Benjamin, Hi Michael,
> > >
> > > regarding commit 51d7d5205d33 ("powerpc: Add smp_mb() to
> > > arch_spin_is_locked()"):
> > >
> > > For the ipc/sem code, I would like to replace the spin_is_locked() with
> > > a smp_load_acquire(), see:
> > >
> > > http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n367
> > >
> > > http://www.ozlabs.org/~akpm/mmots/broken-out/ipc-semc-fix-complex_count-vs-simple-op-race.patch
> > >
> > > To my understanding, I must now add a smp_mb(), otherwise it would be
> > > broken on PowerPC:
> > >
> > > The approach that the memory barrier is added into spin_is_locked()
> > > doesn't work because the code doesn't use spin_is_locked().
> > >
> > > Correct?
> > Right, otherwise you aren't properly ordered. The current powerpc locks provide
> > good protection between what's inside vs. what's outside the lock but not vs.
> > the lock *value* itself, so if, like you do in the sem code, use the lock
> > value as something that is relevant in term of ordering, you probably need
> > an explicit full barrier.

But the problem here is with spin_unlock_wait() (for ll/sc spin_lock) not seeing the
store that makes the lock visibly taken and both threads end up exiting out of sem_lock();
similar scenario to the spin_is_locked commit mentioned above, which is crossing of
locks.

Now that spin_unlock_wait() always implies at least an load-acquire barrier (for both
ticket and qspinlocks, which is still x86 only), we wait on the full critical region.

So this patch takes this locking scheme:

  CPU0			      CPU1
  spin_lock(l)		      spin_lock(L)
  spin_unlock_wait(L)	      if (spin_is_locked(l))
  foo()			 foo()

... and converts it now to:

  CPU0			      CPU1
  complex_mode = true	      spin_lock(l)
  smp_mb()				  <--- do we want a smp_mb() here?
  spin_unlock_wait(l)	      if (!smp_load_acquire(complex_mode))
  foo()			 foo()

We should not be doing an smp_mb() right after a spin_lock(), makes no sense. The
spinlock machinery should guarantee us the barriers in the unorthodox locking cases,
such as this.

Thanks,
Davidlohr