Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation

[Guo Ren <guoren@kernel.org>]

On Tue, Apr 13, 2021 at 6:25 PM Christoph Müllner
<christophm30@gmail.com> wrote:
>
> On Tue, Apr 13, 2021 at 11:37 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Apr 13, 2021 at 11:22:40AM +0200, Christoph Müllner wrote:
> >
> > > > For ticket locks you really only needs atomic_fetch_add() and
> > > > smp_store_release() and an architectural guarantees that the
> > > > atomic_fetch_add() has fwd progress under contention and that a sub-word
> > > > store (through smp_store_release()) will fail the SC.
> > > >
> > > > Then you can do something like:
> > > >
> > > > void lock(atomic_t *lock)
> > > > {
> > > >         u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> > > >         u16 ticket = val >> 16;
> > > >
> > > >         for (;;) {
> > > >                 if (ticket == (u16)val)
> > > >                         break;
> > > >                 cpu_relax();
> > > >                 val = atomic_read_acquire(lock);
> > > >         }
> > > > }
> > > >
> > > > void unlock(atomic_t *lock)
> > > > {
> > > >         u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__);
> > > >         u32 val = atomic_read(lock);
> > > >
> > > >         smp_store_release(ptr, (u16)val + 1);
> > > > }
> > > >
> > > > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal
> > > > on x86 for not being allowed to use a memop on unlock, since its being
> > > > forced into a load-store because of all the volatile, but whatever.
> > >
> > > What about trylock()?
> > > I.e. one could implement trylock() without a loop, by letting
> > > trylock() fail if the SC fails.
> > > That looks safe on first view, but nobody does this right now.
> >
> > Generic code has to use cmpxchg(), and then you get something like this:
> >
> > bool trylock(atomic_t *lock)
> > {
> >         u32 old = atomic_read(lock);
> >
> >         if ((old >> 16) != (old & 0xffff))
> >                 return false;
> >
> >         return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
> > }
>
> This approach requires two loads (atomic_read() and cmpxchg()), which
> is not required.
> Detecting this pattern and optimizing it in a compiler is quite unlikely.
>
> A bit less generic solution would be to wrap the LL/SC (would be
> mandatory in this case)
> instructions and do something like this:
>
> uint32_t __smp_load_acquire_reserved(void*);
> int __smp_store_release_conditional(void*, uint32_t);
>
> typedef union {
>     uint32_t v32;
>     struct {
>         uint16_t owner;
>         uint16_t next;
>     };
> } arch_spinlock_t;
>
> int trylock(arch_spinlock_t *lock)
> {
>     arch_spinlock_t l;
>     int success;
>     do {
>         l.v32 = __smp_load_acquire_reserved(lock);
>         if (l.owner != l.next)
>             return 0;
>         l.next++;
>         success = __smp_store_release_conditional(lock, l.v32);
It's a new semantics v.s cmpxchg, and cmpxchg is come from CAS
instruction to solve some complex scenario.

The primitive of cmpxchg has been widely used in Linux, so I don't
think introducing a new semantics (reserved_conditional) is a good
idea.

Also, from this point: It seems CAS instruction is more suitable for
software compatibility. Although riscv privilege spec chose the LR/SC
and list some bad sides of CAS.

>     } while (!success);
>     return success;
> }
>
> But here we can't tell the compiler to optimize the code between LL and SC...
>
> >
> > That will try and do the full LL/SC loop, because it wants to complete
> > the cmpxchg, but in generic code we have no other option.
> >
> > (Is this what C11's weak cmpxchg is for?)



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/