READ_ONCE_CTRL

[Dmitry Vyukov]

FYI, there is now also READ_ONCE_CTRL:
http://lxr.free-electrons.com/source/include/linux/compiler.h#L258

It is a relaxed load with control-dependency barrier. Since we don't
understand control depenencies, we need to annotate it as
load-acquire.

Andrey, I remember we've hit such case (tsan did not understand
control dependency ordering) and you inserted rmb specifically for
tsan (I've asked you to put it under ifdef and add a comment). We
should start changing such places to use READ_ONCE_CTRL, then we don't
need to insert any tsan-only code.

[Andrey Konovalov]

On Thu, Sep 3, 2015 at 11:30 AM, 'Dmitry Vyukov' via ktsan
<kt...@googlegroups.com> wrote:
> FYI, there is now also READ_ONCE_CTRL:
> http://lxr.free-electrons.com/source/include/linux/compiler.h#L258
>
> It is a relaxed load with control-dependency barrier. Since we don't
> understand control depenencies, we need to annotate it as
> load-acquire.

OK.

> Andrey, I remember we've hit such case (tsan did not understand
> control dependency ordering) and you inserted rmb specifically for
> tsan (I've asked you to put it under ifdef and add a comment). We
> should start changing such places to use READ_ONCE_CTRL, then we don't
> need to insert any tsan-only code.

Yes, this is how we annotated that:

245 if ((atomic_read(&pid->count) == 1) ||
246 atomic_dec_and_test(&pid->count)) {
247 #ifndef CONFIG_KTSAN
248 /* Turn atomic_read into load acquire since ktsan
249 doesn't understand control dependency. */
250 rmb();
251 #endif /* CONFIG_KTSAN */
252 kmem_cache_free(ns->pid_cachep, pid);
253 put_pid_ns(ns);
254 }
255 }

I'm not sure how to fit READ_ONCE_CTRL in here, since the type is atomic_t.

[Dmitry Vyukov]

On second thought, it can break only if compiler/processor does write
speculation. Kernel MM declares that it cannot happen.
So let's leave this code as is.



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Dmitry Vyukov, Software Engineer, dvy...@google.com
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