linux-kernel - Re: [PATCH v4 net-next 1/4] net: core: page_pool: add user cnt preventing pool deletion

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Message-ID: <20190627214317.237e5926@carbon>
Date:   Thu, 27 Jun 2019 21:44:46 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Cc:     davem@...emloft.net, grygorii.strashko@...com, saeedm@...lanox.com,
        leon@...nel.org, ast@...nel.org, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, ilias.apalodimas@...aro.org,
        netdev@...r.kernel.org, daniel@...earbox.net,
        jakub.kicinski@...ronome.com, john.fastabend@...il.com,
        brouer@...hat.com
Subject: Re: [PATCH v4 net-next 1/4] net: core: page_pool: add user cnt
 preventing pool deletion

On Tue, 25 Jun 2019 20:59:45 +0300
Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org> wrote:

> Add user counter allowing to delete pool only when no users.
> It doesn't prevent pool from flush, only prevents freeing the
> pool instance. Helps when no need to delete the pool and now
> it's user responsibility to free it by calling page_pool_free()
> while destroying procedure. It also makes to use page_pool_free()
> explicitly, not fully hidden in xdp unreg, which looks more
> correct after page pool "create" routine.

I don't think that "create" and "free" routines paring looks "more
correct" together.

Maybe we can scale back your solution(?), via creating a page_pool_get()
and page_pool_put() API that can be used by your driver, to keep the
page_pool object after a xdp_rxq_info_unreg() call.  Then you can use
it for two xdp_rxq_info structs, and call page_pool_put() after you
have unregistered both.

The API would basically be:

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index b366f59885c1..691ddacfb5a6 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -357,6 +357,10 @@ static void __warn_in_flight(struct page_pool *pool)
 void __page_pool_free(struct page_pool *pool)
 {
        WARN(pool->alloc.count, "API usage violation");
+
+       if (atomic_read(&pool->user_cnt) != 0)
+               return;
+
        WARN(!ptr_ring_empty(&pool->ring), "ptr_ring is not empty");
 
        /* Can happen due to forced shutdown */
@@ -372,6 +376,19 @@ void __page_pool_free(struct page_pool *pool)
 }
 EXPORT_SYMBOL(__page_pool_free);
 
+void page_pool_put(struct page_pool *pool)
+{
+       if (!atomic_dec_and_test(&pool->user_cnt))
+               __page_pool_free(pool);
+}
+EXPORT_SYMBOL(page_pool_put);
+
+void page_pool_get(struct page_pool *pool)
+{
+       atomic_inc(&pool->user_cnt);
+}
+EXPORT_SYMBOL(page_pool_get);
+


-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer