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Date: Mon, 25 Dec 2023 12:07:36 +0800
From: Changbin Du <changbin.du@huawei.com>
To: Changbin Du <changbin.du@huawei.com>
CC: Luis Chamberlain <mcgrof@kernel.org>, Andrew Morton
	<akpm@linux-foundation.org>, <linux-modules@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, Hui Wang <hw.huiwang@huawei.com>, Xiaoyi Su
	<suxiaoyi@huawei.com>, Jeffrey Hugo <jhugo@codeaurora.org>, Stephen Rothwell
	<sfr@canb.auug.org.au>
Subject: Re: [PATCH] modules: wait do_free_init correctly
Message-ID: <20231225040736.7htp2cfhocbw4hxw@M910t>
References: <20231219141231.2218215-1-changbin.du@huawei.com>
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 <20231220052751.3zcnsnvjk5vf5t7j@M910t>
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On Thu, Dec 21, 2023 at 10:30:37AM +0800, Changbin Du wrote:
> On Wed, Dec 20, 2023 at 06:32:39AM -0800, Luis Chamberlain wrote:
> > On Wed, Dec 20, 2023 at 01:27:51PM +0800, Changbin Du wrote:
> > > On Tue, Dec 19, 2023 at 01:52:03PM -0800, Luis Chamberlain wrote:
> > > > On Tue, Dec 19, 2023 at 12:51:51PM -0800, Andrew Morton wrote:
> > > > > On Tue, 19 Dec 2023 22:12:31 +0800 Changbin Du <changbin.du@huawei.com> wrote:
> > > > > 
> > > > > > The commit 1a7b7d922081 ("modules: Use vmalloc special flag") moves
> > > > > > do_free_init() into a global workqueue instead of call_rcu(). So now
> > > > > > we should wait it via flush_work().
> > > > > 
> > > > > What are the runtime effects of this change?
> > > > 
> > > > Indeed that's needed given how old this culprit commit is:
> > > > 
> > > > git describe --contains 1a7b7d922081
> > > > v5.2-rc1~192^2~5
> > > > 
> > > > Who did this work and for what reason? What triggered this itch?
> > > >
> > > Seems the waiting was introduced by commit ae646f0b9ca ("init: fix false positives
> > > in W+X checking").
> > > 
> > > As what I have observed, mark_readonly() is only invoked by the first user mode
> > > thread function kernel_init(), which is before userspace /init. So is it real
> > > possible we have loaded modules at this point?
> > 
> > Are you saying we don't free any module inits at all then? I asked a lot
> > of questions and your answers seem slim.
> >
> Yes, indeed no module loaded at all before mark_readonly(), at least on my desktop.
> So I think we can just delete this synchronization. I am not sure whether there are
> any historical reasons.
>
I thought about it again, kernel doesn't prevent any drivers from calling
request_module() before init. So it's possible that some particular modules do
behave this way.

I will send an updated one to fix the compilation issue for no CONFIG_MODULES.

-- 
Cheers,
Changbin Du