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


On Tue, 2 Aug 2022, Will Deacon wrote:

> On Mon, Aug 01, 2022 at 12:12:47PM -0400, Mikulas Patocka wrote:
> > 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.
>
> If we need these accesses to be ordered reliably, then we need a CPU barrier
> and that will additionally prevent the compiler reordering. So I still don't
> think we need the barrier() here.

This is x86-specific code. x86 has strong memory ordering, so we only care
about compiler reordering.

We could use smp_rmb() (or smp_load_acquire()) instead of barrier() here,
but smp_rmb() and smp_load_acquire() on x86 is identical to barrier()
anyway.

Mikulas

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