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

["Christoph Müllner" <christophm30@gmail.com>]

On Tue, Apr 13, 2021 at 12:45 PM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> On Tue, Apr 13, 2021 at 12:25:00PM +0200, Christoph Müllner 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:
> > > > 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);
> >     } while (!success);
> >     return success;
> > }
> >
> > But here we can't tell the compiler to optimize the code between LL and SC...
>
> This indeed needs some care. IIUC RISC-V has similar restrictions as arm
> here, no load/store instructions are allowed between LR and SC. You
> can't guarantee that the compiler won't spill some variable onto the
> stack.

RISC-V behaves similar, but the specification is a bit more precise:
To guarantee forward progress, the ("constrained") LL/SC sequence has to
consist of <=16 instructions. Further, the "dynamic code executed between
the LR and SC instructions can only contain instructions from the base “I”
instruction set, excluding loads, stores, backward jumps, taken backward
branches, JALR, FENCE, and SYSTEM instructions".

And GCC generates a backward jump in-between LL and SC.
So we have more than enough reasons, to no do it this way.

>
> BTW, I think the SC doesn't need release semantics above, only the LR
> needs acquire semantics.
>
> --
> Catalin