Em Fri, Jul 03, 2015 at 09:14:46AM +0200, Peter Zijlstra escreveu:
> On Fri, Jul 03, 2015 at 06:21:12AM +0930, Rusty Russell wrote:
> > Looks like Peter Zijlstra is the one to take this fix...
>
> acme is the steward of tools/perf/
>
> > >> diff --git a/tools/perf/util/include/linux/rcupdate.h
> > >> b/tools/perf/util/include/linux/rcupdate.h
> > >> new file mode 100644
> > >> index 0000000..51c0f45
> > >> --- /dev/null
> > >> +++ b/tools/perf/util/include/linux/rcupdate.h
> > >> @@ -0,0 +1,9 @@
> > >> +#ifndef PERF_LINUX_RCUPDATE_H_
> > >> +#define PERF_LINUX_RCUPDATE_H_
> > >> +
> > >> +/* Simple trivial wrappers for now, we don't use RCU in perf user-space
> > >> (yet): */
> > >> +#define WRITE_ONCE(var, val) ((var) = (val))
>
> It looks like perf includes linux/compiler.h so it should already have this.
>
> > >> +#define rcu_assign_pointer(ptr, val) WRITE_ONCE(ptr, val)
>
> That's plain wrong, WRITE_ONCE(*(ptr), (val))
Are you sure?
In the kernel, we have this sequence:
#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
#define smp_store_release(p, v) \
do { \
compiletime_assert_atomic_type(*p); \
smp_mb(); \
ACCESS_ONCE(*p) = (v); \
} while (0)
So, if you go shortcircuiting things you remove that & and that *, no?
I.e. end up with what Rusty suggested.
So, I am trying to keep as much as the semantics of the kernel not to
fall into thse traps...
Will post a RFC soon, if the rain continues preventing me from
running...
- Arnaldo
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