On 27/04/2023 02.57, Yunsheng Lin wrote:
On 2023/4/26 1:15, Jesper Dangaard Brouer wrote:
@@ -609,6 +609,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
recycle_stat_inc(pool, ring_full);
page_pool_return_page(pool, page);
}
+ if (pool->p.flags & PP_FLAG_SHUTDOWN)
+ page_pool_shutdown_attempt(pool);
It seems we have allowed page_pool_shutdown_attempt() to be called
concurrently here, isn't there a time window between atomic_inc_return_relaxed()
and page_pool_inflight() for pool->pages_state_release_cnt, which may cause
double calling of page_pool_free()?
Yes, I think that is correct.
I actually woke up this morning thinking of this case of double freeing,
and this time window. Thanks for spotting and confirming this issue.
Basically: Two concurrent CPUs executing page_pool_shutdown_attempt()
can both end-up seeing inflight equal zero, resulting in both of them
kfreeing the memory (in page_pool_free()) as they both think they are
the last user of PP instance.
I've been thinking how to address this.
This is my current idea:
(1) Atomic variable inc and test (or cmpxchg) that resolves last user race.
(2) Defer free to call_rcu callback to let other CPUs finish.
(3) Might need rcu_read_lock() in page_pool_shutdown_attempt().
--Jesper
