On Wed, 8 May 2024 09:33:43 +0200
Morten Brørup <[email protected]> wrote:
> > What more specifically did you have in mind? READ_ONCE() and
> > WRITE_ONCE()? They give almost no guarantees. Very much relaxed.
>
> The way I read it, they do provide memory ordering guarantees.
>
> Ignore that the kernel's "once" functions operates on words and this RFC
> operates on bits, the behavior is the same. Either there are memory ordering
> guarantees, or there are not.
The kernel's READ_ONCE/WRITE_ONCE are compiler only ordering, i.e only apply to
single CPU.
RTFM memory-barriers.txt..
GUARANTEES
----------
There are some minimal guarantees that may be expected of a CPU:
(*) On any given CPU, dependent memory accesses will be issued in order, with
respect to itself. This means that for:
Q = READ_ONCE(P); D = READ_ONCE(*Q);
the CPU will issue the following memory operations:
Q = LOAD P, D = LOAD *Q
and always in that order. However, on DEC Alpha, READ_ONCE() also
emits a memory-barrier instruction, so that a DEC Alpha CPU will
instead issue the following memory operations:
Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER
Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler
mischief.
(*) Overlapping loads and stores within a particular CPU will appear to be
ordered within that CPU. This means that for:
a = READ_ONCE(*X); WRITE_ONCE(*X, b);
the CPU will only issue the following sequence of memory operations:
a = LOAD *X, STORE *X = b
And for:
WRITE_ONCE(*X, c); d = READ_ONCE(*X);
the CPU will only issue:
STORE *X = c, d = LOAD *X
(Loads and stores overlap if they are targeted at overlapping pieces of
memory).
