Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function

[Marcelo Tosatti]

On Thu, Mar 02, 2023 at 11:42:57AM +0100, David Hildenbrand wrote:
> On 09.02.23 16:01, Marcelo Tosatti wrote:
> > Goal is to have vmstat_shepherd to transfer from
> > per-CPU counters to global counters remotely. For this,
> > an atomic this_cpu_cmpxchg is necessary.
> > 
> > Following the kernel convention for cmpxchg/cmpxchg_local,
> > change ARM's this_cpu_cmpxchg_ helpers to be atomic,
> > and add this_cpu_cmpxchg_local_ helpers which are not atomic.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > 
> > Index: linux-vmstat-remote/arch/arm64/include/asm/percpu.h
> > ===================================================================
> > --- linux-vmstat-remote.orig/arch/arm64/include/asm/percpu.h
> > +++ linux-vmstat-remote/arch/arm64/include/asm/percpu.h
> > @@ -232,13 +232,23 @@ PERCPU_RET_OP(add, add, ldadd)
> >   	_pcp_protect_return(xchg_relaxed, pcp, val)
> >   #define this_cpu_cmpxchg_1(pcp, o, n)	\
> > -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +	_pcp_protect_return(cmpxchg, pcp, o, n)
> >   #define this_cpu_cmpxchg_2(pcp, o, n)	\
> > -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +	_pcp_protect_return(cmpxchg, pcp, o, n)
> >   #define this_cpu_cmpxchg_4(pcp, o, n)	\
> > -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +	_pcp_protect_return(cmpxchg, pcp, o, n)
> >   #define this_cpu_cmpxchg_8(pcp, o, n)	\
> > +	_pcp_protect_return(cmpxchg, pcp, o, n)
> > +
> > +#define this_cpu_cmpxchg_local_1(pcp, o, n)	\
> >   	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +#define this_cpu_cmpxchg_local_2(pcp, o, n)	\
> > +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +#define this_cpu_cmpxchg_local_4(pcp, o, n)	\
> > +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +#define this_cpu_cmpxchg_local_8(pcp, o, n)	\
> > +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> > +
> 
> Call me confused (not necessarily your fault :) ).
> 
> We have cmpxchg_local, cmpxchg_relaxed and cmpxchg. this_cpu_cmpxchg_local_*
> now calls ... *drumroll* ... cmpxchg_relaxed.
> IIUC, cmpxchg_local is only guaranteed to be atomic WRO the current CPU
> (especially, protection against interrupts when the operation is implemented
> using multiple instructions). We do have a generic implementation that
> disables/enables interrupts.
>
> IIUC, cmpxchg_relaxed an atomic update without any memory ordering
> guarantees (in contrast to cmpxchg, cmpxchg_acquire, cmpxchg_acquire). We
> default to arch_cmpxchg if we don't have arch_cmpxchg_relaxed. arch_cmpxchg
> defaults to arch_cmpxchg_local, if not supported.
> 
> 
> Naturally I wonder:
> 
> (a) Should these new variants be rather called
>     this_cpu_cmpxchg_relaxed_* ?

No: it happens that on ARM-64 cmpxchg_local == cmpxchg_relaxed.

See cf10b79a7d88edc689479af989b3a88e9adf07ff.

> (b) Should these new variants rather call the "_local" variant?

They probably should. But this patchset maintains the current behaviour
of this_cpu_cmpxch (for this_cpu_cmpxch_local), which was:

 #define this_cpu_cmpxchg_1(pcp, o, n)  \
-       _pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+       _pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_2(pcp, o, n)  \
-       _pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+       _pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_4(pcp, o, n)  \
-       _pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+       _pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_8(pcp, o, n)  \
+       _pcp_protect_return(cmpxchg, pcp, o, n)


Thanks.