netdev - Re: [PATCH RFC net-next/mm V1 1/3] page_pool: Remove workqueue in new shutdown scheme

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Message-ID: <733c4402-7aea-07a8-de98-184d88fc17f0@redhat.com>
Date:   Thu, 27 Apr 2023 20:29:39 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
        linux-mm@...ck.org, Mel Gorman <mgorman@...hsingularity.net>
Cc:     brouer@...hat.com, lorenzo@...nel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        bpf@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org
Subject: Re: [PATCH RFC net-next/mm V1 1/3] page_pool: Remove workqueue in new
 shutdown scheme


On 27/04/2023 12.47, Jesper Dangaard Brouer wrote:
> 
> 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().
> 

I think I found a more simply approach (adjustment patch attached).
That avoids races and any call_rcu callbacks.

Will post a V2.

--Jesper
View attachment "02-fix-race" of type "text/plain" (4504 bytes)