linux-kernel - Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI

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Message-ID: <561774D2.3050002@kyup.com>
Date:	Fri, 9 Oct 2015 11:03:30 +0300
From:	Nikolay Borisov <kernel@...p.com>
To:	Hillf Danton <hillf.zj@...baba-inc.com>
Cc:	'linux-kernel' <linux-kernel@...r.kernel.org>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-fsdevel@...r.kernel.org,
	SiteGround Operations <operations@...eground.com>,
	vbabka@...e.cz, gilad@...yossef.com, mgorman@...e.de,
	linux-mm@...ck.org, Marian Marinov <mm@...com>
Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts
 disabled and page-draining IPI

On 10/09/2015 10:37 AM, Hillf Danton wrote:
>>>> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio)
>>>>  			if (bio->bi_error)
>>>>  				buffer_io_error(bh);
>>>>  		} while ((bh = bh->b_this_page) != head);
>>>> -		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
>>>>  		local_irq_restore(flags);
>>>
>>> What if it takes 100ms to unlock after IRQ restored?
>>
>> I'm not sure I understand in what direction you are going? Care to
>> elaborate?
>>
> Your change introduces extra time cost the lock waiter has to pay in
> the case that irq happens before the lock is released.

[CC filesystem and mm people. For reference the thread starts here:
 http://thread.gmane.org/gmane.linux.kernel/2056996 ]

Right, I see what you mean and it's a good point but when doing the
patches I was striving for correctness and starting a discussion, hence
the RFC. In any case I'd personally choose correctness over performance
always ;).

As I'm not an fs/ext4 expert and have added the relevant parties (please
use reply-all from now on so that the thread is not being cut in the
middle) who will be able to say whether it impact is going to be that
big. I guess in this particular code path worrying about this is prudent
as writeback sounds like a heavily used path.

Maybe the problem should be approached from a different angle e.g.
drain_all_pages and its reliance on the fact that the IPI will always be
delivered in some finite amount of time? But what if a cpu with disabled
interrupts is waiting on the task issuing the IPI?

> 
>>>> +		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
>>>>  		if (!under_io) {
>>>>  #ifdef CONFIG_EXT4_FS_ENCRYPTION
>>>>  			if (ctx)
>>>> --
>>>> 2.5.0
>>>
> 
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