netdev - Re: [PATCH net] page_pool: fix inconsistency for page_pool_ring

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Date: Sat, 27 May 2023 15:56:26 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <davem@...emloft.net>, <pabeni@...hat.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Jesper Dangaard Brouer <hawk@...nel.org>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, Eric Dumazet
	<edumazet@...gle.com>, Lorenzo Bianconi <lorenzo@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net] page_pool: fix inconsistency for
 page_pool_ring_[un]lock()

On 2023/5/27 3:34, Jakub Kicinski wrote:
> On Mon, 22 May 2023 11:17:14 +0800 Yunsheng Lin wrote:
>> page_pool_ring_[un]lock() use in_softirq() to decide which
>> spin lock variant to use, and when they are called in the
>> context with in_softirq() being false, spin_lock_bh() is
>> called in page_pool_ring_lock() while spin_unlock() is
>> called in page_pool_ring_unlock(), because spin_lock_bh()
>> has disabled the softirq in page_pool_ring_lock(), which
>> causes inconsistency for spin lock pair calling.
>>
>> This patch fixes it by returning in_softirq state from
>> page_pool_producer_lock(), and use it to decide which
>> spin lock variant to use in page_pool_producer_unlock().
>>
>> As pool->ring has both producer and consumer lock, so
>> rename it to page_pool_producer_[un]lock() to reflect
>> the actual usage. Also move them to page_pool.c as they
>> are only used there, and remove the 'inline' as the
>> compiler may have better idea to do inlining or not.
>>
>> Fixes: 7886244736a4 ("net: page_pool: Add bulk support for ptr_ring")
>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> 
> I just realized now while doing backports that the Fixes tag is
> incorrect here. The correct Fixes tag is:
> 
> Fixes: 542bcea4be86 ("net: page_pool: use in_softirq() instead")
> 
> Before that we used in_serving_softirq() which was perfectly fine.

>From the comment around in_serving_softirq() and in_softirq(),
you are probably right as in_serving_softirq() is always false
no matter if bh is enabled or disabled.

> This explains the major mystery of how such a serious bug would survive
> for 10+ releases... it didn't, it wasn't there :) It only came in 6.3.
> We can't change the tag now but at least the universe makes sense again.

Yes, it makes more sense now:)

> .
>