Re: bit fields && data tearing

Paul E. McKenney
2014-09-07T16:21:46Z

On Sat, Sep 06, 2014 at 10:07:22PM -0700, James Bottomley wrote:
> On Thu, 2014-09-04 at 21:06 -0700, Paul E. McKenney wrote:
> > On Thu, Sep 04, 2014 at 10:47:24PM -0400, Peter Hurley wrote:
> > > Hi James,
> > >
> > > On 09/04/2014 10:11 PM, James Bottomley wrote:
> > > > On Thu, 2014-09-04 at 17:17 -0700, Paul E. McKenney wrote:
> > > >> +And there are anti-guarantees:
> > > >> +
> > > >> + (*) These guarantees do not apply to bitfields, because compilers often
> > > >> + generate code to modify these using non-atomic read-modify-write
> > > >> + sequences. Do not attempt to use bitfields to synchronize parallel
> > > >> + algorithms.
> > > >> +
> > > >> + (*) Even in cases where bitfields are protected by locks, all fields
> > > >> + in a given bitfield must be protected by one lock. If two fields
> > > >> + in a given bitfield are protected by different locks, the compiler's
> > > >> + non-atomic read-modify-write sequences can cause an update to one
> > > >> + field to corrupt the value of an adjacent field.
> > > >> +
> > > >> + (*) These guarantees apply only to properly aligned and sized scalar
> > > >> + variables. "Properly sized" currently means "int" and "long",
> > > >> + because some CPU families do not support loads and stores of
> > > >> + other sizes. ("Some CPU families" is currently believed to
> > > >> + be only Alpha 21064. If this is actually the case, a different
> > > >> + non-guarantee is likely to be formulated.)
> > > >
> > > > This is a bit unclear. Presumably you're talking about definiteness of
> > > > the outcome (as in what's seen after multiple stores to the same
> > > > variable).
> > >
> > > No, the last conditions refers to adjacent byte stores from different
> > > cpu contexts (either interrupt or SMP).
> > >
> > > > The guarantees are only for natural width on Parisc as well,
> > > > so you would get a mess if you did byte stores to adjacent memory
> > > > locations.
> > >
> > > For a simple test like:
> > >
> > > struct x {
> > > long a;
> > > char b;
> > > char c;
> > > char d;
> > > char e;
> > > };
> > >
> > > void store_bc(struct x *p) {
> > > p->b = 1;
> > > p->c = 2;
> > > }
> > >
> > > on parisc, gcc generates separate byte stores
> > >
> > > void store_bc(struct x *p) {
> > > 0: 34 1c 00 02 ldi 1,ret0
> > > 4: 0f 5c 12 08 stb ret0,4(r26)
> > > 8: 34 1c 00 04 ldi 2,ret0
> > > c: e8 40 c0 00 bv r0(rp)
> > > 10: 0f 5c 12 0a stb ret0,5(r26)
> > >
> > > which appears to confirm that on parisc adjacent byte data
> > > is safe from corruption by concurrent cpu updates; that is,
> > >
> > > CPU 0 | CPU 1
> > > |
> > > p->b = 1 | p->c = 2
> > > |
> > >
> > > will result in p->b = 1 && p->c = 2 (assume both values
> > > were 0 before the call to store_bc()).
> >
> > What Peter said. I would ask for suggestions for better wording, but
> > I would much rather be able to say that single-byte reads and writes
> > are atomic and that aligned-short reads and writes are also atomic.
> >
> > Thus far, it looks like we lose only very old Alpha systems, so unless
> > I hear otherwise, I update my patch to outlaw these very old systems.
>
> This isn't universally true according to the architecture manual. The
> PARISC CPU can make byte to long word stores atomic against the memory
> bus but not against the I/O bus for instance. Atomicity is a property
> of the underlying substrate, not of the CPU. Implying that atomicity is
> a CPU property is incorrect.

OK, fair point.

But are there in-use-for-Linux PARISC memory fabrics (for normal memory,
not I/O) that do not support single-byte and double-byte stores?

Thanx, Paul