Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Stafford Horne]

On Tue, Mar 30, 2021 at 06:08:40PM +0200, Peter Zijlstra wrote:
> On Tue, Mar 30, 2021 at 11:13:55AM +0800, Guo Ren wrote:
> > On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote:
> > > > u32 a = 0x55aa66bb;
> > > > u16 *ptr = &a;
> > > >
> > > > CPU0                       CPU1
> > > > =========             =========
> > > > xchg16(ptr, new)     while(1)
> > > >                                     WRITE_ONCE(*(ptr + 1), x);
> > > >
> > > > When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock.
> > >
> > > Then I think your LL/SC is broken.
> > >
> > > That also means you really don't want to build super complex locking
> > > primitives on top, because that live-lock will percolate through.
> 
> > Do you mean the below implementation has live-lock risk?
> > +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> > +{
> > +       u32 old, new, val = atomic_read(&lock->val);
> > +
> > +       for (;;) {
> > +               new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> > +               old = atomic_cmpxchg(&lock->val, val, new);
> > +               if (old == val)
> > +                       break;
> > +
> > +               val = old;
> > +       }
> > +       return old;
> > +}
> 
> That entirely depends on the architecture (and cmpxchg() impementation).
> 
> There are a number of cases:
> 
>  * architecture has cmpxchg() instruction (x86, s390, sparc, etc.).
> 
>   - architecture provides fwd progress (x86)
>   - architecture requires backoff for progress (sparc)
> 
>  * architecture does not have cmpxchg, and implements it using LL/SC.
> 
>   and here things get *really* interesting, because while an
>   architecture can have LL/SC fwd progress, that does not translate into
>   cmpxchg() also having the same guarantees and all bets are off.
> 
> The real bummer is that C can do cmpxchg(), but there is no way it can
> do LL/SC. And even if we'd teach C how to do LL/SC, it couldn't be
> generic because architectures lacking it can't emulate it using
> cmpxchg() (there's a fun class of bugs there).
> 
> So while the above code might be the best we can do in generic code,
> it's really up to the architecture to make it work.

I just want to chime in here, there may be a better spot in the thread to
mention this but, for OpenRISC I did implement some generic 8/16-bit xchg code
which I have on my todo list somwhere to replace the other generic
implementations like that in mips.

  arch/openrisc/include/asm/cmpxchg.h

The idea would be that architectures just implement these methods:

  long cmpxchg_u32(*ptr,old,new)
  long xchg_u32(*ptr,val)

Then the rest of the generic header would implement cmpxchg.

-Stafford