On Tue, Jun 07, 2016 at 02:41:44PM +0200, Hannes Frederic Sowa wrote:
> On 07.06.2016 09:15, Peter Zijlstra wrote:
> >>
> >> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> >> index 147ae8ec836f..a4d0a99de04d 100644
> >> --- a/Documentation/memory-barriers.txt
> >> +++ b/Documentation/memory-barriers.txt
> >> @@ -806,6 +806,41 @@ out-guess your code. More generally, although READ_ONCE() does force
> >> the compiler to actually emit code for a given load, it does not force
> >> the compiler to use the results.
> >>
> >> +In addition, control dependencies apply only to the then-clause and
> >> +else-clause of the if-statement in question. In particular, it does
> >> +not necessarily apply to code following the if-statement:
> >> +
> >> + q = READ_ONCE(a);
> >> + if (q) {
> >> + WRITE_ONCE(b, p);
> >> + } else {
> >> + WRITE_ONCE(b, r);
> >> + }
> >> + WRITE_ONCE(c, 1); /* BUG: No ordering against the read from "a". */
> >> +
> >> +It is tempting to argue that there in fact is ordering because the
> >> +compiler cannot reorder volatile accesses and also cannot reorder
> >> +the writes to "b" with the condition. Unfortunately for this line
> >> +of reasoning, the compiler might compile the two writes to "b" as
> >> +conditional-move instructions, as in this fanciful pseudo-assembly
> >> +language:
>
> I wonder if we already guarantee by kernel compiler settings that this
> behavior is not allowed by at least gcc.
>
> We unconditionally set --param allow-store-data-races=0 which should
> actually prevent gcc from generating such conditional stores.
>
> Am I seeing this correct here?
In this case, the store to "c" is unconditional, so pulling it forward
would not generate a data race. However, the compiler is still prohibited
from pulling it forward because it is not allowed to reorder volatile
references. So, yes, the compiler cannot reorder, but for a different
reason.
Some CPUs, on the other hand, can do this reordering, as Will Deacon
pointed out earlier in this thread.
Thanx, Paul
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