linux-kernel - Re: [PATCH] scatterlist.h: Change CONFIG_DEBUG_SG for ifdef statement in sg_set

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Date:	Sun, 3 Aug 2014 07:02:25 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Nick Krause <xerofoify@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] scatterlist.h: Change CONFIG_DEBUG_SG for ifdef
 statement in sg_set_bf

On Sun, Aug 03, 2014 at 12:31:30AM -0400, Nick Krause wrote:
> On Sat, Aug 2, 2014 at 11:59 PM, Mateusz Guzik <mguzik@...hat.com> wrote:
> > On Sat, Aug 02, 2014 at 10:56:13PM -0400, Nicholas Krause wrote:
> >> This changes the ifdef statement in sg_set_bg to !CONFIG_DEBUG_SG in order
> >> to avoid a bug with xhci dequence/enquence functions.
> >>
> >> Signed-off-by: Nicholas Krause <xerofoify@...il.com>
> >> ---
> >>  include/linux/scatterlist.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> >> index adae88f..62de7b3 100644
> >> --- a/include/linux/scatterlist.h
> >> +++ b/include/linux/scatterlist.h
> >> @@ -111,7 +111,7 @@ static inline struct page *sg_page(struct scatterlist *sg)
> >>  static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
> >>                             unsigned int buflen)
> >>  {
> >> -#ifdef CONFIG_DEBUG_SG
> >> +#ifdef !CONFIG_DEBUG_SG
> >>       BUG_ON(!virt_addr_valid(buf));
> >>  #endif
> >
> > Have you tried compiling this? IIRC you said you would compile your
> > stuff, what hapened to that?
> >
> > What exactly were you trying to achieve? Did this BUG_ON detect a
> > problem on your system and now you are trying to silence it?
> >
> > The change would be wrong even if it compiled since it would just
> > execute the assertion only when debug is disabled.
> >
> > --
> > Mateusz Guzik
> This is the mailing theme I am getting this from,[xhci] kernel BUG at
> include/linux/scatterlist.h:115.
> I hope this answers your question about the BUG_ON and yes I did
> compile check it with make
> M=include/. I also checked usb and usb net directories too.

So how have you verified it tests you change? Why didn't you perform a
full build?

This is a syntax error, I suggest you read up about C preprocessor.

Your change attempts to flip the condition. Now virt_addr_valid(buf) is
tested only with debug disabled. When you enable debug it is suddenly
not tested - definitely does not make sense.

I'm assuming you are talking about https://lkml.org/lkml/2014/7/30/810

If you actually read the thread you will note:
> Looks like I either need specify valid addresses to sg_set_buf(), or
> just make the unit test depend on !CONFIG_DEBUG_SG. 

1. It is acknowleged the problem is in the caller
2. There is a suggestion to ensure that the UNIT TEST is not executed if
CONFIG_DEBUG_SG  is enabled (this part was shortened to "!CONFIG_DEBUG_SG"
but nobody claims you can use this in if/if[n]def statements)

UNIT TEST as in the thingy which resulted in passing down a buffer
failing on this BUG_ON.

There is no suggestion to do anything with sg_set_buf itself.

You were advised several times to find a simpler project. Also people
noted that a "beginner kernel programmer" actually means "seasoned
programmer learning the kernel". It is clear you are not a seasoned
programmer, so why do you insist on doing kernel work?

I can only recommend you play with userspace programs for now. These are
much easier to debug and experiment with, not to mention have a lot lower
entry point.

-- 
Mateusz Guzik
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