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SubjectRe: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation
On 11/29, Paul E. McKenney wrote:
>
> On Thu, Nov 30, 2006 at 04:57:14AM +0300, Oleg Nesterov wrote:
> > (the same patch + comments from Paul)
> >
> With the addition of a comment for the smp_mb() at the beginning of
> synchronize_qrcu(), shown below:
>
> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Thanks!

> /*
> * The following memory barrier is needed to ensure that
> * and subsequent freeing of data elements previously
> * removed is seen by other CPUs after the wait completes.
> */

I think we have another reason for mb(), but I can't suggest a clear
comment.

struct data {
...
int in_use;
...
}

void free_data(struct data *p)
{
BUG_ON(p->in_use);
kfree(p);
}

struct data *DATA;

Reader:

qrcu_read_lock();
data = rcu_dereference(DATA);

data->in_use = 1;
do_something(data);
data->in_use = 0;

qrcu_read_unlock();

Writer:

old = DATA;
DATA = alloc_new_data();

synchronize_qrcu();
free_data(old);

qrcu_read_unlock() does (implicit) mb() on reader's side, but we must pair
it on our side, otherwise we can't be sure (of course, _only_ in theory) we
are seeing all the changes (->in_use == 0) made by the reader.

> Hmmm... Now I am wondering if the memory barriers inherent in the
> __wait_event() suffice for this last barrier... :-/ Thoughts?
>
> > + smp_mb();

Fastpath skips __wait_event(), and it is possible that the reader does
lock/unlock between the first 'mb()' and 'if (atomic_read() == 1)'.

Oleg.

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