Linux Virtualization - Jul 2020 - [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation

On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will at kernel.org>
wrote:
> On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
> > On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
> > > -#define read_barrier_depends() __asm__
__volatile__("mb": : :"memory")
> > > +#define __smp_load_acquire(p)                                   
\
> > > +({                                                              
\
> > > +   __unqual_scalar_typeof(*p) ___p1 =                           
\
> > > +           (*(volatile typeof(___p1) *)(p));                    
\
> > > +   compiletime_assert_atomic_type(*p);                          
\
> > > +   ___p1;                                                       
\
> > > +})
> >
> > Sorry if I'm being thick, but doesn't this need a barrier
after the
> > volatile access to provide the acquire semantic?
> >
> > IIUC prior to this commit alpha would have used the asm-generic
> > __smp_load_acquire, i.e.
> >
> > | #ifndef __smp_load_acquire
> > | #define __smp_load_acquire(p)                                       
\
> > | ({                                                                  
\
> > |         __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);           
\
> > |         compiletime_assert_atomic_type(*p);                         
\
> > |         __smp_mb();                                                 
\
> > |         (typeof(*p))___p1;                                          
\
> > | })
> > | #endif
I also have a question that I didn't dare ask when the same
code came up before (I guess it's also what's in the kernel today):

With the cast to 'typeof(*p)' at the end, doesn't that mean we
lose the effect of __unqual_scalar_typeof() again, so any "volatile"
pointer passed into __READ_ONCE_SCALAR() or
__smp_load_acquire() still leads to a volatile load of the original
variable, plus another volatile access to ___p1 after
spilling it to the stack as a non-volatile variable?

I hope I'm missing something obvious here.

        Arnd

On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann
wrote:
> On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will at kernel.org>
wrote:
> > On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
> > > On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
> > > > -#define read_barrier_depends() __asm__
__volatile__("mb": : :"memory")
> > > > +#define __smp_load_acquire(p)                              
\
> > > > +({                                                         
\
> > > > +   __unqual_scalar_typeof(*p) ___p1 =                      
\
> > > > +           (*(volatile typeof(___p1) *)(p));               
\
> > > > +   compiletime_assert_atomic_type(*p);                     
\
> > > > +   ___p1;                                                  
\
> > > > +})
> > >
> > > Sorry if I'm being thick, but doesn't this need a barrier
after the
> > > volatile access to provide the acquire semantic?
> > >
> > > IIUC prior to this commit alpha would have used the asm-generic
> > > __smp_load_acquire, i.e.
> > >
> > > | #ifndef __smp_load_acquire
> > > | #define __smp_load_acquire(p)                                  
\
> > > | ({                                                             
\
> > > |         __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);      
\
> > > |         compiletime_assert_atomic_type(*p);                    
\
> > > |         __smp_mb();                                            
\
> > > |         (typeof(*p))___p1;                                     
\
> > > | })
> > > | #endif
> 
> I also have a question that I didn't dare ask when the same
> code came up before (I guess it's also what's in the kernel today):
> 
> With the cast to 'typeof(*p)' at the end, doesn't that mean we
> lose the effect of __unqual_scalar_typeof() again, so any
"volatile"
> pointer passed into __READ_ONCE_SCALAR() or
> __smp_load_acquire() still leads to a volatile load of the original
> variable, plus another volatile access to ___p1 after
> spilling it to the stack as a non-volatile variable?
Not sure I follow you here, but I can confirm that what you're worried
about doesn't happen for the usual case of a pointer-to-volatile scalar.

For example, ignoring dependency ordering:

unsigned long foo(volatile unsigned long *p)
{
	return smp_load_acquire(p) + 1;
}

Ends up looking like:

	unsigned long ___p1 = *(const volatile unsigned long *)p;
	smp_mb();
	(volatile unsigned long)___p1;

My understanding is that casting a non-pointer type to volatile doesn't
do anything, so we're good.

On the other hand, you can still cause the stack reload if you use volatile
pointers to volatile:

volatile unsigned long *bar(volatile unsigned long * volatile *ptr)
{
	return READ_ONCE(*ptr);
}

but this is pretty weird code, I think.

Will

On Thu, Jul 2, 2020 at 1:18 PM Will Deacon <will at kernel.org>
wrote:
> On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote:
> > On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will at kernel.org>
wrote:
> > > On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
> Not sure I follow you here, but I can confirm that what you're worried
> about doesn't happen for the usual case of a pointer-to-volatile
scalar.
>
> For example, ignoring dependency ordering:
>
> unsigned long foo(volatile unsigned long *p)
> {
>         return smp_load_acquire(p) + 1;
> }
>
> Ends up looking like:
>
>         unsigned long ___p1 = *(const volatile unsigned long *)p;
>         smp_mb();
>         (volatile unsigned long)___p1;
>
> My understanding is that casting a non-pointer type to volatile doesn't
> do anything, so we're good.
Right, I mixed up the correct

        (typeof(*p))___p;

with the incorrect

       *typeof(p)&___p;

which would dereference a volatile pointer and cause the
problem.

The code is all fine then.

    Arnd