Re: bit fields && data tearing

Paul E. McKenney
2014-09-05T04:06:45Z

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.

Thanx, Paul