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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 14, 2021 at 02:55:57PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 14, 2021 at 08:39:33PM +0800, Guo Ren wrote:

> > > + * 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.

Anyway, if we can mandate that atomic acquire/release is RCsc, then yes
we can do that but then we also need:

#define smp_mb__after_spinlock()	smp_mb__after_atomic()

But currently atomic acquire/release is not RCsc (at the very least
Power does RCtso -- and power gets to deal with any and all pain caused
by that).