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From: Andrew Morton on 12 May 2010 03:40
On Wed, 12 May 2010 15:20:51 +0800 Miao Xie <miaox(a)cn.fujitsu.com> wrote:

> @@ -985,6 +984,7 @@ repeat:
> * for the read-side.
> */
> while (ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
> + task_unlock(tsk);
> if (!task_curr(tsk))
> yield();
> goto repeat;

Oh, I meant to mention that. No yield()s, please. Their duration is
highly unpredictable. Can we do something more deterministic here?

Did you consider doing all this with locking? get_mems_allowed() does
mutex_lock(current->lock)?

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From: Andrew Morton on 12 May 2010 13:50
On Wed, 12 May 2010 17:00:45 +0800
Miao Xie <miaox(a)cn.fujitsu.com> wrote:

> on 2010-5-12 12:32, Andrew Morton wrote:
> > On Wed, 12 May 2010 15:20:51 +0800 Miao Xie <miaox(a)cn.fujitsu.com> wrote:
> >
> >> @@ -985,6 +984,7 @@ repeat:
> >> * for the read-side.
> >> */
> >> while (ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
> >> + task_unlock(tsk);
> >> if (!task_curr(tsk))
> >> yield();
> >> goto repeat;
> >
> > Oh, I meant to mention that. No yield()s, please. Their duration is
> > highly unpredictable. Can we do something more deterministic here?
>
> Maybe we can use wait_for_completion().

That would work.

> >
> > Did you consider doing all this with locking? get_mems_allowed() does
> > mutex_lock(current->lock)?
>
> do you means using a real lock(such as: mutex) to protect mempolicy and mem_allowed?

yes.

> It may cause the performance regression, so I do my best to abstain from using a real
> lock.

Well, the code as-is is pretty exotic with lots of open-coded tricky
barriers - it's best to avoid inventing new primitives if possible.
For example, there's no lockdep support for this new "lock".

mutex_lock() is pretty quick - basically a simgle atomic op. How
frequently do these operations occur?

The code you have at present is fairly similar to sequence locks. I
wonder if there's some way of (ab)using sequence locks for this.
seqlocks don't have lockdep support either...

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From: Andrew Morton on 13 May 2010 15:20
On Thu, 13 May 2010 14:16:33 +0800
Miao Xie <miaox(a)cn.fujitsu.com> wrote:

> >
> > The code you have at present is fairly similar to sequence locks. I
> > wonder if there's some way of (ab)using sequence locks for this.
> > seqlocks don't have lockdep support either...
> >
>
> We can't use sequence locks here, because the read-side may read the data
> in changing, but it can't put off cleaning the old bits.

I don't understand that sentence. Can you expand on it please?
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