Re: [PATCH] locking/atomic: atomic: Use arch_atomic_{read,set} in generic atomic ops

[Jules Maselbas]

Hi Peter,

On Fri, Jan 27, 2023 at 03:34:33PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 27, 2023 at 02:49:46PM +0100, Jules Maselbas wrote:
> > Hi Peter,
> > 
> > On Fri, Jan 27, 2023 at 12:18:13PM +0100, Peter Zijlstra wrote:
> > > On Thu, Jan 26, 2023 at 06:33:54PM +0100, Jules Maselbas wrote:
> > > 
> > > > @@ -58,9 +61,11 @@ static inline int generic_atomic_fetch_##op(int i, atomic_t *v)		\
> > > >  static inline void generic_atomic_##op(int i, atomic_t *v)		\
> > > >  {									\
> > > >  	unsigned long flags;						\
> > > > +	int c;								\
> > > >  									\
> > > >  	raw_local_irq_save(flags);					\
> > > > -	v->counter = v->counter c_op i;					\
> > > > +	c = arch_atomic_read(v);					\
> > > > +	arch_atomic_set(v, c c_op i);					\
> > > >  	raw_local_irq_restore(flags);					\
> > > >  }
> > > 
> > > This and the others like it are a bit sad, it explicitly dis-allows the
> > > compiler from using memops and forces a load-store.
> > Good point, I don't know much about atomic memops but this is indeed a
> > bit sad to prevent such instructions to be used.
> 
> Depends on the platform, x86,s390 etc. have then, RISC like things
> typically don't.
> 
> > > The alternative is writing it like:
> > > 
> > > 	*(volatile int *)&v->counter c_op i;
> > I wonder if it could be possible to write something like:
> > 
> >         *(volatile int *)&v->counter += i;
> 
> Should work, but give it a try, see what it does :-)
> 

I've made a quick test on godbolt[1] and I don't see a major difference
between the old version and the new version I propose. I am not very
familiar with both x86 and s390 architecture and I might have missed an
option for gcc to automagically generate "memops" instructions.

[1] https://godbolt.org/z/nrvvMs9b6

>From my understanding s390 has instructions to read a value from memory
and add a value, but still needs to be written by another instruction.

x86 is not using the generic atomic code, but has its own implementation
of atomic memory operations using lock {add,...} instructions.

The goal of the proposed patch is to make the generic code more correct:
| I don't think that's true; without READ_ONCE() the compiler could (but
| is very unlikely to) read multiple times, and that could cause problems.
explained by Mark Rutland here:
https://lore.kernel.org/lkml/Y71LoCIl+IFdy9D8@FVFF77S0Q05N/

I still have some open questions:

 - Maybe in SMP the generic_atomic_* functions should use READ_ONCE
instead of arch_atomic_read, since only the "once" part is what is
needed, and the atomicity is done by the cmpxchg.

 - I have the feeling that in non-SMP we do not need the atomicity at all.


Thanks
-- Jules

> > I also noticed that GCC has some builtin/extension to do such things,
> > __atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this
> > can be used in the kernel.
> 
> On a per-architecture basis only, the C/C++ memory model does not match
> the Linux Kernel memory model so using the compiler to generate the
> atomic ops is somewhat tricky and needs architecture audits.
> 
> 
> 
>