Re: __{read,write}_once

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Mon, Nov 11, 2019 at 11:51:26AM -0500, Mouse wrote:
> >>> (2) Use uninterruptible memory operations in per CPU memory,
> >>> aggregate passively on demand.
> > Problem is that (2) is natively only present on CISC platforms in
> > general.  Most RISC platforms can't do RMW in one instruction.
> 
> (2) says "uninterruptible", not "one instruction", though I'm not sure
> how large the difference is in practice.  (Also, some CISC platforms
> provide atomic memory RMW operations only under annoying restrictions;
> for example, I think the VAX has only three sorts of RMW memory
> accesses that are atomic with respect to other processors: ADAWI
> (16-bit-aligned 16-bit add), BB{SS,CC}I (test-and-{set/clear} single
> bits), and {INS,REM}Q{H,T}I (queue insert/remove at head/tail).)

The point here is that we really don't want to have bus locked
instructions for per-CPU counters. It would defeat the point of using
per-CPU counters in first place to a large degree. Uninterruptible means
exactly that, there is a clear before and after state and no interrupts
can happen in between. I don't know about ARM and friends for how
expensive masking interrupts is. It is quite expensive on x86. RMW
instructions are the other simple option for implementing them. (3)
would be the high effort version, doing RAS for kernel code as well.

Joerg