linux-kernel - Re: [PATCH] modules: fix livelock in add_unformed

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Date:   Mon, 13 May 2019 10:37:03 -0400
From:   Barret Rhoden <brho@...gle.com>
To:     Prarit Bhargava <prarit@...hat.com>, Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        David Arcari <darcari@...hat.com>
Subject: Re: [PATCH] modules: fix livelock in add_unformed_module()

Hi -

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().

Barret