linux-kernel - Re: [PATCH] mm: forward declare struct rcuwait together with rcuwait_wake

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Message-ID: <CAJuCfpE-DPBp-0b0908FHvguSo_PqxE2LdUXvBiMLu1UCd5icQ@mail.gmail.com>
Date: Tue, 16 Sep 2025 15:04:21 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: forward declare struct rcuwait together with rcuwait_wake_up()

On Tue, Sep 16, 2025 at 6:59 AM Thomas Weißschuh
<thomas.weissschuh@...utronix.de> wrote:
>
> At the point of the forward declaration of rcuwait_wake_up()
> in mmap_lock.h 'struct rcuwait' may have not yet been declared,
> leading to compiler errors.
>
> Add an explicit forward declaration for the struct.
>
> Fixes: 75404e07663b ("mm: move mmap/vma locking logic into specific files")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> ---
> This doesn't seem to break any in-tree code right now.
> I stumbled upon it while building a series for the next cycle.
> Instead of putting this fix into my series and spamming all the mm
> maintainers with it, maybe this could be part of the last mm bugfix pull
> for this cycle.

`struct rcuwait` is defined inside include/linux/types.h and
mmap_lock.h includes that file. Could you please explain in more
detail what exactly failed when you were building it?

> ---
>  include/linux/mmap_lock.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 11a078de9150df1beff4f0bfb16e199333767614..9792dd4fff0ff73829833aae8ea3229a31757d61 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -3,6 +3,7 @@
>  #define _LINUX_MMAP_LOCK_H
>
>  /* Avoid a dependency loop by declaring here. */
> +struct rcuwait;
>  extern int rcuwait_wake_up(struct rcuwait *w);
>
>  #include <linux/lockdep.h>
>
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250916-mm-rcuwait-03c5fe95f36d
>
> Best regards,
> --
> Thomas Weißschuh <thomas.weissschuh@...utronix.de>
>