Re: dcache locking question

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Mar 16, 2019 at 07:20:20PM -0700, James Bottomley wrote:
> On Sat, 2019-03-16 at 17:50 -0700, Paul E. McKenney wrote:
> [...]
> >  I -have- seen stores of constant values be torn, but not stores of
> > runtime-variable values and not loads.  Still, such tearing is
> > permitted, and including the READ_ONCE() is making it easier for
> > things like thread sanitizers.  In addition, the READ_ONCE() makes it
> > clear that the value being loaded is unstable, which can be
> > useful documentation.
> 
> Um, just so I'm clear, because this assumption permeates all our code: 
> load or store tearing can never occur if we're doing load or store of a
> 32 bit value which is naturally aligned.  Where naturally aligned is
> within the gift of the CPU to determine but which the compiler or
> kernel will always ensure for us unless we pack the structure or
> deliberately misalign the allocation.

Wait a sec; are there any 64bit architectures where the same is not
guaranteed for dereferencing properly aligned void **?

If that's the case, I can think of quite a few places that are rather
dubious, and I don't see how READ_ONCE() could help in those - e.g.
if an architecture only has 32bit loads, rcu list traversals are
not going to be doable without one hell of an extra headache.