Re: [PATCH v4 1/2] introduce test_bit_acquire and use it in wait_on_bit

[Mikulas Patocka]

On Mon, 1 Aug 2022, Will Deacon wrote:

> On Mon, Aug 01, 2022 at 06:42:15AM -0400, Mikulas Patocka wrote:
> 
> > Index: linux-2.6/arch/x86/include/asm/bitops.h
> > ===================================================================
> > --- linux-2.6.orig/arch/x86/include/asm/bitops.h	2022-08-01 12:27:43.000000000 +0200
> > +++ linux-2.6/arch/x86/include/asm/bitops.h	2022-08-01 12:27:43.000000000 +0200
> > @@ -203,8 +203,10 @@ arch_test_and_change_bit(long nr, volati
> >  
> >  static __always_inline bool constant_test_bit(long nr, const volatile unsigned long *addr)
> >  {
> > -	return ((1UL << (nr & (BITS_PER_LONG-1))) &
> > +	bool r = ((1UL << (nr & (BITS_PER_LONG-1))) &
> >  		(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
> > +	barrier();
> > +	return r;
> 
> Hmm, I find it a bit weird to have a barrier() here given that 'addr' is
> volatile and we don't need a barrier() like this in the definition of
> READ_ONCE(), for example.

gcc doesn't reorder two volatile accesses, but it can reorder non-volatile
accesses around volatile accesses.

The purpose of the compiler barrier is to make sure that the non-volatile 
accesses that follow test_bit are not reordered by the compiler before the 
volatile access to addr.

> > Index: linux-2.6/include/linux/wait_bit.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/wait_bit.h	2022-08-01 12:27:43.000000000 +0200
> > +++ linux-2.6/include/linux/wait_bit.h	2022-08-01 12:27:43.000000000 +0200
> > @@ -71,7 +71,7 @@ static inline int
> >  wait_on_bit(unsigned long *word, int bit, unsigned mode)
> >  {
> >  	might_sleep();
> > -	if (!test_bit(bit, word))
> > +	if (!test_bit_acquire(bit, word))
> >  		return 0;
> >  	return out_of_line_wait_on_bit(word, bit,
> >  				       bit_wait,
> 
> Yet another approach here would be to leave test_bit as-is and add a call to
> smp_acquire__after_ctrl_dep() since that exists already -- I don't have
> strong opinions about it, but it saves you having to add another stub to
> x86.

It would be the same as my previous patch with smp_rmb() that Linus didn't 
like. But I think smp_rmb (or smp_acquire__after_ctrl_dep) would be 
correct here.

> Will

Mikulas