On Fri, Nov 25, 2016 at 9:28 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
On Fri, Nov 25, 2016 at 5:17 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>> IE, they are strictly stronger than {READ,WRITE}_ONCE().
No, they are strictly bullshit.
Stop this idiocy. We went through this once already.
> Uh, so, READ/WRITE_ONCE are non-atomic now. I missed that.
No.
READ/WRITE_ONCE() are atomic *WHEN*THAT*IS*POSSIBLE*. So for something
that fits in a register, it will read it in one atomic access. For
something that fits in a register and is _possible_ to write
atomically, it will do so.
But sometimes it's not going to be atomic. We do not for a moment try
to make multi-word accesses be atomic. Not even if you could try to
use some magic cmpxchg16b thing. It's not "atomic" in that sense: it
will be doing multiple accesses.
Similarly, if you try to write a 8- or 16-bit word on alpha with
WRITE_ONCE() or you try to do other things, you have what's coming to
you.
And they just force some "copy to stable storage" when it isn't (ie a
"memcpy()" is not necessarily a single access and might be done as
multiple overlapping reads, but the end result is stable).
So trying to make anything else out of them is f*cking stupid.
READ_ONCE() and friends do the right thing. Trying to limit them is
*wrong*, because the restrictions would simply make them less useful.
And trying to make up something new is pointless and stupid.
So leave this code alone. Don't add some stupid "SINGLE_LOAD()" crap.
That's just moronic. READ_ONCE() is that, and so much more.
Linus
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