Re: kernel/trace/ring_buffer.c:412:29-47: WARNING: casting value returned by memory allocation function to (struct buffer_page *) is useless.

[Julia Lawall]

On Fri, 31 Jul 2026, Steven Rostedt wrote:

> On Sat, 01 Aug 2026 08:10:14 +0800
> kernel test robot <lkp@xxxxxxxxx> wrote:
>
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   8ba098e6b6ff0db8edf28528d1552be261af30d4
> > commit: b1e7a590a0133606d3efd41aee38cdeac630b52f ring-buffer: Add helper functions for allocations
> > date:   8 months ago
> > config: openrisc-randconfig-r062-20260801 (https://download.01.org/0day-ci/archive/20260801/202608010824.vc7vATf0-lkp@xxxxxxxxx/config)
> > compiler: or1k-linux-gcc (GCC) 9.5.0
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Fixes: b1e7a590a013 ("ring-buffer: Add helper functions for allocations")
> > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202608010824.vc7vATf0-lkp@xxxxxxxxx/
> >
> > cocci warnings: (new ones prefixed by >>)
> > >> kernel/trace/ring_buffer.c:412:29-47: WARNING: casting value returned by memory allocation function to (struct buffer_page *) is useless.
> > >> kernel/trace/ring_buffer.c:408:31-57: WARNING: casting value returned by memory allocation function to (struct ring_buffer_per_cpu *) is useless.
>
> No, whatever Coccinelle script that reported this is broken.
>
> The typecast is in a macro. The macro is adding the typecast to make
> sure that what it is being assigned to is of the proper type.
>
> Hence, it is very *useful*!

Thanks for the report.  I will remove the warning in the case of a macro
body.

Is there some reason why it is not an inline function?

julia

>
> -- Steve
>
>
> >
> > vim +412 kernel/trace/ring_buffer.c
> >
> >    403
> >    404	/*
> >    405	 * For best performance, allocate cpu buffer data cache line sized
> >    406	 * and per CPU.
> >    407	 */
> >  > 408	#define alloc_cpu_buffer(cpu) (struct ring_buffer_per_cpu *)		\
> >    409		kzalloc_node(ALIGN(sizeof(struct ring_buffer_per_cpu),		\
> >    410				   cache_line_size()), GFP_KERNEL, cpu_to_node(cpu));
> >    411
> >  > 412	#define alloc_cpu_page(cpu) (struct buffer_page *)			\
> >    413		kzalloc_node(ALIGN(sizeof(struct buffer_page),			\
> >    414				   cache_line_size()), GFP_KERNEL, cpu_to_node(cpu));
> >    415
> >
> > --
> > 0-DAY CI Kernel Test Service
> > https://github.com/intel/lkp-tests/wiki
>
>