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: <56177FC3.4050506@kyup.com>
Date:	Fri, 9 Oct 2015 11:50:11 +0300
From:	Nikolay Borisov <kernel@...p.com>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
Cc:	mm@...com, linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	adilger.kernel@...ger.ca, linux-kernel@...r.kernel.org,
	operations@...eground.com, tytso@....edu
Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts
 disabled and page-draining IPI



On 10/09/2015 11:41 AM, Gilad Ben-Yossef wrote:
> On Oct 8, 2015 18:31, "Nikolay Borisov" <kernel@...p.com> wrote:
>>
>> Currently when bios are being finished in ext4_finish_bio this is done by
>> first disabling interrupts and then acquiring a bit_spin_lock.
> ...
>>
>> To fix the situation this patch changes the order in which the
>> bit_spin_lock and interrupts disabling occcurs. The exepected
>> effect is that even if a core is spinning on the bitlock it will
>> have its interrupts enabled, thus being able to respond to IPIs.
> 
> Are you sure this spin lock is never taken from interrupt context?

Good point.

I think indeed this fix is wrong as this lock is actually taken from
soft irq context as evident from this thread, which prompted me to write
the patch in the first place:
http://permalink.gmane.org/gmane.linux.kernel/2056730


> If it does an interrupt occurring after the lock is taken and before
> interrupts are disabled can deadlock .
> 
> Gilad
> 
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