Re: [PATCH] fs: Remove obsolete logic in i_size_read/write

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jul 16, 2025 at 09:44:31PM +0800, Alex wrote:
> On Wed, Jul 16, 2025 at 9:41 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Jul 16, 2025 at 09:28:29PM +0800, Alex wrote:
> > > On Wed, Jul 16, 2025 at 9:12 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > >
> > > > On Wed, Jul 16, 2025 at 08:53:04PM +0800, Alex wrote:
> > > > > The logic is used to protect load/store tearing on 32 bit platforms,
> > > > > for example, after i_size_read returned, there is no guarantee that
> > > > > inode->size won't be changed. Therefore, READ/WRITE_ONCE suffice, which
> > > > > is already implied by smp_load_acquire/smp_store_release.
> > > >
> > > > Sorry, what?  The problem is not a _later_ change, it's getting the
> > > > upper and lower 32bit halves from different values.
> > > >
> > > > Before: position is 0xffffffff
> > > > After: position is 0x100000000
> > > > The value that might be returned by your variant: 0x1ffffffff.
> > >
> > > I mean the sequence lock here is used to only avoid load/store tearing,
> > > smp_load_acquire/smp_store_release already protects that.
> >
> > Why do you think that?  You're wrong, but it'd be useful to understand
> > what misled you into thinking that.
> 
> smp_load_acquire/smp_store_release implies READ_ONCE/WRITE_ONCE,
> and READ_ONCE/WRITE_ONCE avoid load/store tearing.
> 
> What am I missing here?

They only avoid tearing for sizes <= word size.  If you have a 32-bit
CPU, they cannot avoid tearing for 64-bit loads/stores.