linux-kernel - Re: [PATCH] Revert "module, async: async_synchronize

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Date:   Fri, 28 Jan 2022 09:39:12 +0200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Igor Pylypiv <ipylypiv@...gle.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>, Tejun Heo <tj@...nel.org>,
        linux-modules@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Changyuan Lyu <changyuanl@...gle.com>
Subject: Re: [PATCH] Revert "module, async: async_synchronize_full() on module
 init iff async is used"

On Fri, Jan 28, 2022 at 1:40 AM Igor Pylypiv <ipylypiv@...gle.com> wrote:
>
> This reverts commit 774a1221e862b343388347bac9b318767336b20b.

Whee. That's from early 2013, more than 9 years ago.

> This works when modprobe thread is calling async_schedule(), but it
> does not work if module dispatches init code to a worker thread which
> then calls async_schedule().

Hmm.

You make a good argument:

> Commit 21c3c5d28007 ("block: don't request module during elevator init")
> fixed the deadlock issue which the reverted commit 774a1221e862 ("module,
> async: async_synchronize_full() on module init iff async is used") tried
> to fix.
>
> Since commit 0fdff3ec6d87 ("async, kmod: warn on synchronous
> request_module() from async workers") synchronous module loading
> from async is not allowed.

This does seem to imply that limiting it to PF_USED_ASYNC is largely
pointless, at least for the originally stated reason of deadlocks with
other module loading.

However, we've done this for *so* long that I wonder if there might be
situations that have ended up depending on the lack of synchronization
for pure performance reasons.

If *this* module loading process started the async work, then we'd
wait for it, but what if there's other async work that was started by
others? This revert would now make us wait for that async work too,
and that might be a big deal slowing things down at boot time.

Looking at it, this is all under the 'module_mutex', so I guess we are
already single-threaded at least wrt loading other modules, so the
amount of unrelated async work going on is presumably fairly low and
that isn't an issue.

Anyway, I think this patch is the right thing to do, but just the fact
that we've avoided that async wait for so long makes me a bit nervous
about fallout from the revert.

Comments? Maybe this is a "just apply it, see if somebody screams" situation?

                   Linus