linux-kernel - Re: [PATCH 2/2] module: add support to avoid duplicates early on load

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Message-ID: <CAHk-=wg7ihygotpO9x5a6QJO5oAom9o91==L_Kx-gUHvRYuXiQ@mail.gmail.com>
Date:   Mon, 29 May 2023 21:55:15 -0400
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Petr Pavlu <petr.pavlu@...e.com>, gregkh@...uxfoundation.org,
        rafael@...nel.org, song@...nel.org, lucas.de.marchi@...il.com,
        christophe.leroy@...roup.eu, peterz@...radead.org, rppt@...nel.org,
        dave@...olabs.net, willy@...radead.org, vbabka@...e.cz,
        mhocko@...e.com, dave.hansen@...ux.intel.com,
        colin.i.king@...il.com, jim.cromie@...il.com,
        catalin.marinas@....com, jbaron@...mai.com,
        rick.p.edgecombe@...el.com, yujie.liu@...el.com, david@...hat.com,
        tglx@...utronix.de, hch@....de, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pmladek@...e.com, prarit@...hat.com,
        lennart@...ttering.net
Subject: Re: [PATCH 2/2] module: add support to avoid duplicates early on load

On Mon, May 29, 2023 at 11:18 AM Johan Hovold <johan@...nel.org> wrote:
>
> I took a closer look at some of the modules that failed to load and
> noticed a pattern in that they have dependencies that are needed by more
> than one device.

Ok, this is a "maybe something like this" RFC series of two patches -
one trivial one to re-organize things a bit so that we can then do the
real one which uses a filter based on the inode pointer to return an
"idempotent return value" for module loads that share the same inode.

It's entirely untested, and since I'm on the road I'm going to not
really be able to test it. It compiles for me, and the code looks
fairly straightforward, but it's probably buggy.

It's very loosely based on Luis' attempt,  but it
 (a) is internal to module loading
 (b) uses a reliable cookie
 (c) doesn't leave the cookie around randomly for later
 (d) has seen absolutely no testing

Put another way: if somebody wants to play with this, please treat it
as a starting point, not the final thing. You might need to debug
things, and fix silly mistakes.

The idea is to just have a simple hash list of currently executing
module loads, protected by a trivial spinlock. Every module loader
adds itself to the right hash list, and if they were the *first* one
(ie no other pending module loads for that inode), will actually do
the module load.

Everybody who *isn't* the first one will just wait for completion and
return the same error code that the first one returned.

This is technically bogus. The first one might fail due to arguments.
So the cookie shouldn't be just the inode, it should be the inode and
a hash of the arguments or something like that. But it is what it is,
and apart from possible show-stopper bugs this is no worse than the
failed "exclusive write deny" attempt. IOW - maybe worth trying?

And if *that* didn't sell people on this patch series, I don't know
what will. I should be in marketing! Two drink minimums, here I come!

               Linus

View attachment "0001-module-split-up-finit_module-into-init_module_from_f.patch" of type "text/x-patch" (2888 bytes)

View attachment "0002-modules-catch-concurrent-module-loads-take-two.patch" of type "text/x-patch" (3300 bytes)