linux-kernel - Re: [PATCH v4 33/40] sh: mach-x3proto: optimize ilsel

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Date: Fri, 21 Jun 2024 07:30:22 -0700
From: Yury Norov <yury.norov@...il.com>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: linux-kernel@...r.kernel.org,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Rich Felker <dalias@...c.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	linux-sh@...r.kernel.org, Alexey Klimov <alexey.klimov@...aro.org>,
	Bart Van Assche <bvanassche@....org>, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Shtylyov <s.shtylyov@....ru>
Subject: Re: [PATCH v4 33/40] sh: mach-x3proto: optimize ilsel_enable()

On Fri, Jun 21, 2024 at 10:48:44AM +0200, John Paul Adrian Glaubitz wrote:
> Hi Yury,
> 
> thanks for your patch!
> 
> On Thu, 2024-06-20 at 10:56 -0700, Yury Norov wrote:
> > Simplify ilsel_enable() by using find_and_set_bit().
> > 
> > Geert also pointed the bug in the old implementation:
> > 
> > 	I don't think the old code worked as intended: the first time
> > 	no free bit is found, bit would have been ILSEL_LEVELS, and
> > 	test_and_set_bit() would have returned false, thus terminating
> > 	the loop, and continuing with an out-of-range bit value? Hence
> > 	to work correctly, bit ILSEL_LEVELS of ilsel_level_map should
> > 	have been initialized to one?  Or am I missing something?
> > 
> > The new code does not have that issue.
> > 
> > CC: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > ---
> >  arch/sh/boards/mach-x3proto/ilsel.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/sh/boards/mach-x3proto/ilsel.c b/arch/sh/boards/mach-x3proto/ilsel.c
> > index f0d5eb41521a..35b585e154f0 100644
> > --- a/arch/sh/boards/mach-x3proto/ilsel.c
> > +++ b/arch/sh/boards/mach-x3proto/ilsel.c
> > @@ -8,6 +8,7 @@
> >   */
> >  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> >  
> > +#include <linux/find_atomic.h>
> >  #include <linux/init.h>
> >  #include <linux/kernel.h>
> >  #include <linux/module.h>
> > @@ -99,8 +100,8 @@ int ilsel_enable(ilsel_source_t set)
> >  	}
> >  
> >  	do {
> > -		bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS);
> > -	} while (test_and_set_bit(bit, &ilsel_level_map));
> > +		bit = find_and_set_bit(&ilsel_level_map, ILSEL_LEVELS);
> > +	} while (bit >= ILSEL_LEVELS);
> >  
> >  	__ilsel_enable(set, bit);
> 
> I will need to take a closer look at the whole code in ilsel_enable() to understand what's
> happening here. If Geert's explanation is correct, it sounds more like you're fixing a bug
> and saying you're optimizing the function in the patch subject would sound more like an
> euphemism.
> 
> Also, I think we should add a Fixes tag if possible in case your patch fixes an actual bug.
> 
> I will have a closer look over the weekend.

Hi John,

The problem is that if the ilsel_level_map if dense, the @bit
will be set to ILSEL_LEVELS. The following test_and_set_bit()
will therefore access a bit beyond the end of bitmap. Which in
turn is undef.

I'm not familiar to the subsystem as whole, so I can't say if it's
ever possible to have the ilsel_level_map all set. If you take a
look that would be great.

If this series will not move, the fix for this code would be:

  do {
          bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS);
  } while (bit >= ILSEL_LEVELS || test_and_set_bit(bit, &ilsel_level_map));

It would work, but because find_first_zero_bit() is not designed to
work correctly in concurrent environment, it may trigger KCSAN and/or
return something non-relevant. See cover letter on this series for
details.

Thanks,
Yury