On 05/12, Peter Zijlstra wrote:
>
> On Sat, 2007-05-12 at 20:04 +0400, Oleg Nesterov wrote:
> >
> > this code roughly does (the only reader does unlock)
> >
> > READER WRITER
> >
> > readers = 0; state = 1;
> > wmb(); wmb();
> > CHECK(state != 0) CHECK(readers == 0)
> >
> > We need to ensure that we can't miss both CHECKs. Either reader
> > should see RW_MUTEX_READER_SLOW, o writer sees "readers == 0"
> > and does not sleep.
> >
> > In that case both barriers should be converted to smp_mb(). There
> > was a _long_ discussion about STORE-MB-LOAD behaviour, and experts
> > seem to believe everething is ok.
>
> Ah, but note that both those CHECK()s have a rmb(), so that ends up
> being:
>
> READER WRITER
>
> readers = 0; state = 1;
> wmb(); wmb();
>
> rmb(); rmb();
> if (state != 0) if (readers == 0)
>
> and a wmb+rmb is a full mb, right?
I used to think the same, but this is wrong: wmb+rmb != mb. wmb+rmb
doesn't provide LOAD,STORE or STORE,LOAD ordering.
for example,
LOAD;
rmb(); wmb();
STORE;
it is still possible that STORE comes before LOAD. At least this
is my understanding.
Oleg.
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