[RFC][PATCH] locking: Generic ticket-lock

[Peter Zijlstra]

On Wed, Apr 14, 2021 at 08:39:33PM +0800, Guo Ren wrote:

> I've tested it on csky SMP*4 hw (860) & riscv SMP*4 hw (c910) and it's okay.

W00t :-)

> Hope you can keep
> typedef struct {
>         union {
>                 atomic_t lock;
>                 struct __raw_tickets {
> #ifdef __BIG_ENDIAN
>                         u16 next;
>                         u16 owner;
> #else
>                         u16 owner;
>                         u16 next;
> #endif
>                 } tickets;
>         };
> } arch_spinlock_t;
> 
> Using owner & next is much more readable.

That almost doubles the line-count of the thing ;-)


> > + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence
> > + * uses atomic_fetch_add() which is SC to create an RCsc lock.

This ^^^ then vvv

> > +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> > +{
> > +       u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> atomic_fetch_add_acquire ?

Then we must rely on the arch to implement RCsc atomics. And I for one
can never tell wth Risc-V actually does.

> > +static __always_inline int ticket_is_locked(arch_spinlock_t *lock)
> > +{
> > +       u32 val = atomic_read(lock);
> > +
> > +       return ((val >> 16) != (val & 0xffff));
> I perfer:
> return !arch_spin_value_unlocked(READ_ONCE(*lock));
> > +}

> > +}
> > +
> > +static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
> > +{
> > +       return !ticket_is_locked(&lock);
> Are you sure to let ticket_is_locked->atomic_read(lock) again, the
> lock has contained all information?
> 
> return lock.tickets.owner == lock.tickets.next;

Yeah, I wrote then the wrong way around. Couldn't be bothered to go back
when I figured it out.

> > +
> > +static __always_inline int ticket_is_contended(arch_spinlock_t *lock)
> > +{
> > +       u32 val = atomic_read(lock);
> > +
> > +       return (s16)((val >> 16) - (val & 0xffff)) > 1;
> How big-endian ?

How not? Endian-ness only matters when you go poke at sub-words, which
the above does not. Only ticket_unlock() does and cares about that.