linux-kernel - Re: kfifo must

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Message-Id: <20101019221242.c732e1fa.akpm@linux-foundation.org>
Date:	Tue, 19 Oct 2010 22:12:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Stefani Seibold <stefani@...bold.net>
Subject: Re: kfifo must_check warning

On Tue, 19 Oct 2010 15:10:48 -0700 Randy Dunlap <randy.dunlap@...cle.com> wrote:

> In 2.6.36-rc8, I see this build warning:
> 
> drivers/char/n_gsm.c: In function 'gsm_dlci_alloc':
> drivers/char/n_gsm.c:1580: warning: ignoring return value of '__kfifo_must_check_helper', declared with attribute warn_unused_result
> 
> The helper seems to be getting in the way (?).  The driver code does this:
> 
> 	if (kfifo_alloc(&dlci->_fifo, 4096, GFP_KERNEL) < 0) {
> 		kfree(dlci);
> 		return NULL;
> 	}
> 
> Should the driver code be doing something else?
> or should the kfifo_alloc() macro be checking the result of the helper?
> 

A gcc bug, I'd say.  The code looks OK and my gcc doesn't warn.

Perhaps see if you can find some code transformation in n_gsm.c which
makes it go away?  Add a new local variable or something.


I did see a probably unrelated bug in there though. 
__kfifo_must_check_helper() coerces its arg into an `unsigned int' and
returns an unsigned int.  Consequently if kfifo_alloc() tries to return
-EINVAL, the above-quoted code won't detect the error: it will see
-EINVAL as a large, positive unsigned value.

I don't see a simple fix for that apart from creating several flavours
of __kfifo_must_check_helper() and carefully going through each
instance and using the one which takes (and returns) the correct type.

A suitable temporary 2.6.37 patch would bee to just disable
__kfifo_must_check_helper():

--- a/include/linux/kfifo.h~a
+++ a/include/linux/kfifo.h
@@ -171,11 +171,7 @@ struct kfifo_rec_ptr_2 __STRUCT_KFIFO_PT
 	}
 
 
-static inline unsigned int __must_check
-__kfifo_must_check_helper(unsigned int val)
-{
-	return val;
-}
+#define __kfifo_must_check_helper(x) (x)
 
 /**
  * kfifo_initialized - Check if the fifo is initialized
_

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