On Mon, Oct 06, 2014 at 02:25:09AM +0200, Oleg Nesterov wrote:
> Yes, and the comments ;)
>
> I showed this patch only to complete the discussion, I am not going to
> send it now.
Fair enough :-)
> But thanks for the review!
>
> > > +static void kthread_kill(struct task_struct *k, struct kthread *kthread)
> > > +{
> > > + smp_mb__before_atomic();
> >
> > test_bit isn't actually an atomic op so this barrier is 'wrong'. If you
> > need an MB there smp_mb() it is.
>
> Hmm. I specially checked Documentation/memory-barriers.txt,
>
> (*) smp_mb__before_atomic();
> (*) smp_mb__after_atomic();
>
> These are for use with atomic (such as add, subtract, increment and
> decrement) functions that don't return a value, especially when used for
> reference counting. These functions do not imply memory barriers.
>
> These are also used for atomic bitop functions that do not return a
> value (such as set_bit and clear_bit).
> ^^^^^^^^^^^^^^^^^^^^^
>
> Either you or memory-barriers.txt should be fixed ;)
Its in there, just not explicitly. All those functions listed are
read-modify-write ops, test_bit() is not, its just a read. But yes I
suppose we could make that more explicit.
Also test_bit() obviously does return a value, so it doesn't fall in the
{set,clear}_bit() class.
Does the change below clarify things?
> > > + if (test_bit(KTHREAD_WANTS_SIGNAL, &kthread->flags)) {
> > > + unsigned long flags;
> > > + bool kill = true;
> > > +
> > > + if (lock_task_sighand(k, &flags)) {
> >
> > Since we do the double test thing here, with the set side also done
> > under the lock, so we really need a barrier above?
>
> Yes, otherwise set_kthread_wants_signal() can miss a signal. And note
> that the 2nd check is only needed to ensure that we can not race
> with set_kthread_wants_signal(false).
>
> BUT!!! I have to admit that I simply do not know if there is any arch
>
> set_bit(&word, X);
> test_bit(&word, Y);
>
> which actually needs mb() in between, the word is the same. Probably
> not.
DEC Alpha? Wasn't it the problem there that dependencies didn't actually
work as expected?
Added Paul to Cc.
---
Documentation/memory-barriers.txt | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)
diff --git a/Documentation/memory-barriers.txt
b/Documentation/memory-barriers.txt
index 22a969cdd476..0d97c99ad957 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1594,12 +1594,9 @@ CPU from reordering them.
(*) smp_mb__before_atomic();
(*) smp_mb__after_atomic();
- These are for use with atomic (such as add, subtract, increment and
- decrement) functions that don't return a value, especially when used for
- reference counting. These functions do not imply memory barriers.
-
- These are also used for atomic bitop functions that do not return a
- value (such as set_bit and clear_bit).
+ These are for use with atomic/bitop (r-m-w) functions that don't return
+ a value (eg. atomic_{add,sub,inc,dec}(), {set,clear}_bit()). These
+ functions do not imply memory barriers.
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count:
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