linux-kernel - Re: [PATCH] efibc: avoid stack overflow warning

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Message-ID: <20160430201449.GL2839@codeblueprint.co.uk>
Date:	Sat, 30 Apr 2016 21:14:49 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"Compostella, Jeremy" <jeremy.compostella@...el.com>,
	Ingo Molnar <mingo@...nel.org>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] efibc: avoid stack overflow warning

On Fri, 29 Apr, at 07:48:31PM, Arnd Bergmann wrote:
> gcc complains about a newly added file for the EFI Bootloader Control:
> 
> drivers/firmware/efi/efibc.c: In function 'efibc_set_variable':
> drivers/firmware/efi/efibc.c:53:1: error: the frame size of 2272 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
> 
> The problem is the declaration of a local variable of type
> struct efivar_entry, which is by itself larger than the warning
> limit of 1024 bytes.
> 
> We know that the reboot notifiers are not called from a deep stack,
> so this is not an actual bug, but we should still try to rework
> the code to avoid the warning. We also know that reboot notifiers
> are never run concurrently on multiple CPUs, so there is no problem
> in just making the variable 'static'.
 
I assumed reboot notifiers were guaranteed to be non-concurrent too
but having dug into the callers of kernel_reboot(), I couldn't find
any kind of mutual exclusion.

How/where is this guaranteed?