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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

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

-- 
Catalin