Re: [PATCH] modules: fix livelock in add_unformed_module()

From: Jessica Yu <jeyu@kernel.org>
To: Prarit Bhargava <prarit@redhat.com>
Cc: Barret Rhoden <brho@google.com>,
	linux-kernel@vger.kernel.org,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	David Arcari <darcari@redhat.com>
Subject: Re: [PATCH] modules: fix livelock in add_unformed_module()
Date: Tue, 28 May 2019 16:47:02 +0200	[thread overview]
Message-ID: <20190528144702.GA3459@linux-8ccs> (raw)
In-Reply-To: <e5f7f37b-5c99-f9de-61ce-5b3394caf0d2@redhat.com>

+++ Prarit Bhargava [28/05/19 10:30 -0400]:
>
>
>On 5/22/19 1:08 PM, Prarit Bhargava wrote:
>>
>>
>> On 5/13/19 10:37 AM, Barret Rhoden wrote:
>>> Hi -
>>>
>>
>> Hey Barret, my apologies for not getting back to you earlier.  I got caught up
>> in something that took me away from this issue.
>>
>>> On 5/13/19 7:23 AM, Prarit Bhargava wrote:
>>> [snip]
>>>> A module is loaded once for each cpu.
>>>
>>> Does one CPU succeed in loading the module, and the others fail with EEXIST?
>>>
>>>> My follow-up patch changes from wait_event_interruptible() to
>>>> wait_event_interruptible_timeout() so the CPUs are no longer sleeping and can
>>>> make progress on other tasks, which changes the return values from
>>>> wait_event_interruptible().
>>>>
>>>> https://marc.info/?l=linux-kernel&m=155724085927589&w=2
>>>>
>>>> I believe this also takes your concern into account?
>>>
>>> That patch might work for me, but I think it papers over the bug where the check
>>> on old->state that you make before sleeping (was COMING || UNFORMED, now !LIVE)
>>> doesn't match the check to wake up in finished_loading().
>>>
>>> The reason the issue might not show up in practice is that your patch basically
>>> polls, so the condition checks in finished_loading() are only a quicker exit.
>>>
>>> If you squash my patch into yours, I think it will cover that case. Though if
>>> polling is the right answer here, it also raises the question of whether or not
>>> we even need finished_loading().
>>>
>>
>> The more I look at this I think you're right.  Let me do some additional testing
>> with your patch + my original patch.
>>
>
>I have done testing on arm64, s390x, ppc64le, ppc64, and x86 and have not seen
>any issues.
>
>Jessica, how would you like me to proceed?  Would you like an updated patch with
>Signed-off's from both Barret & myself?

Hi Prarit,

A freshly sent patch with the squashed changes and Signed-off's would
be great. Thank you both for testing and looking into this!

Jessica