linux-kernel - Re: [PATCH] branch-check: fix long->int truncation when profiling branches

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Message-ID: <20180604165024.3be78887@gandalf.local.home>
Date:   Mon, 4 Jun 2018 16:50:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] branch-check: fix long->int truncation when profiling
 branches

On Wed, 30 May 2018 08:19:22 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> The function __builtin_expect returns long type (see the gcc
> documentation), and so do macros likely and unlikely. Unfortunatelly, when
> CONFIG_PROFILE_ANNOTATED_BRANCHES is selected, the macros likely and
> unlikely expand to __branch_check__ and __branch_check__ truncates the

Nice catch.

> long type to int. This unintended truncation may cause bugs in various
> kernel code (we found a bug in dm-writecache because of it), so it's

I'm curious to what that bug was.

> better to fix __branch_check__ to return long.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> Cc: stable@...r.kernel.org

Anyway, I can pull this in my tree and test it.

-- Steve

> 
> ---
>  include/linux/compiler.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6/include/linux/compiler.h
> ===================================================================
> --- linux-2.6.orig/include/linux/compiler.h	2018-02-26 20:34:17.000000000 +0100
> +++ linux-2.6/include/linux/compiler.h	2018-05-30 14:11:53.000000000 +0200
> @@ -21,7 +21,7 @@ void ftrace_likely_update(struct ftrace_
>  #define unlikely_notrace(x)	__builtin_expect(!!(x), 0)
>  
>  #define __branch_check__(x, expect, is_constant) ({			\
> -			int ______r;					\
> +			long ______r;					\
>  			static struct ftrace_likely_data		\
>  				__attribute__((__aligned__(4)))		\
>  				__attribute__((section("_ftrace_annotated_branch"))) \