linux-kernel - Re: [PATCH v2] atomic: Fix bugs in 'fetch_or()' and rename it to 'xchg

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Date:	Wed, 16 Mar 2016 09:14:44 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] atomic: Fix bugs in 'fetch_or()' and rename it to
 'xchg_or()'


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> > index 6c502cb13c95..7bc5297bcca8 100644
> > --- a/include/linux/atomic.h
> > +++ b/include/linux/atomic.h
> > @@ -549,22 +549,28 @@ static inline int atomic_dec_if_positive(atomic_t *v)
> >  #endif
> >  
> >  /**
> > - * fetch_or - perform *ptr |= mask and return old value of *ptr
> > - * @ptr: pointer to value
> > + * xchg_or - perform *ptr |= mask atomically and return old value of *ptr
> > + * @ptr: pointer to value (cmpxchg() compatible integer pointer type)
> >   * @mask: mask to OR on the value
> >   *
> > - * cmpxchg based fetch_or, macro so it works for different integer types
> > + * cmpxchg() based, it's a macro so it works for different integer types.
> >   */
> > -#ifndef fetch_or
> > -#define fetch_or(ptr, mask)						\
> > -({	typeof(*(ptr)) __old, __val = *(ptr);				\
> > +#ifndef xchg_or
> > +# define xchg_or(ptr, mask)						\
> > +({									\
> > +	typeof(ptr)  _ptr  = (ptr);					\
> 
> Can we add a comment above to prevent from future mistakes with cmpxchg
> variables aliasing?
> 
> I'm suprised that GCC doesn't warn about that actually.

Yeah, so in the perf tooling build we do have -Wshadow to catch such mishaps,
but not in the main kernel build.

... and yes, if I add it via the patch below the bug gets warned about:

 include/linux/atomic.h:561:15: note: shadowed declaration is here
   typeof(ptr)  __ptr  = (ptr);     \
               ^
 kernel/sched/core.c:332:11: note: in expansion of macro ‘xchg_or’
   return !(xchg_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLA

... but I also get a ton of other warnings, just when building a single 
kernel/sched/core.o file:

 ./arch/x86/include/asm/bitops.h:396:28: warning: declaration of ‘ffs’ shadows a built-in function [-Wshadow]
 ./arch/x86/include/asm/bitops.h:396:28: warning: declaration of ‘ffs’ shadows a built-in function [-Wshadow]
 include/linux/jiffies.h:422:60: warning: declaration of ‘jiffies’ shadows a global declaration [-Wshadow]
 ./arch/x86/include/asm/io_apic.h:187:54: warning: declaration of ‘apic’ shadows a global declaration [-Wshadow]
 ./arch/x86/include/asm/bitops.h:396:28: warning: declaration of ‘ffs’ shadows a built-in function [-Wshadow]
 include/linux/jiffies.h:422:60: warning: declaration of ‘jiffies’ shadows a global declaration [-Wshadow]
 ./arch/x86/include/asm/io_apic.h:187:54: warning: declaration of ‘apic’ shadows a global declaration [-Wshadow]
 include/linux/kernel.h:750:12: warning: declaration of ‘_min1’ shadows a previous local [-Wshadow]
 include/linux/kernel.h:750:12: warning: declaration of ‘_min1’ shadows a previous local [-Wshadow]
 include/linux/kernel.h:751:12: warning: declaration of ‘_min2’ shadows a previous local [-Wshadow]
 kernel/sched/sched.h:308:43: warning: declaration of ‘down’ shadows a global declaration [-Wshadow]
 kernel/sched/sched.h:308:60: warning: declaration of ‘up’ shadows a global declaration [-Wshadow]
 kernel/sched/auto_group.h:44:55: warning: declaration of ‘init_task’ shadows a global declaration [-Wshadow]
 kernel/sched/core.c:635:20: warning: declaration of ‘down’ shadows a global declaration [-Wshadow]
 kernel/sched/core.c:635:37: warning: declaration of ‘up’ shadows a global declaration [-Wshadow]

and yes, I'd say most of these are signatures of sloppy macros and sloppy variable 
names - but it would be a ton of work to eliminate these warnings.

Thanks,

	Ingo

==============>

 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index 7b3ecdcdc6c1..14b0ce82f2b0 100644
--- a/Makefile
+++ b/Makefile
@@ -776,6 +776,9 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=date-time)
 # use the deterministic mode of AR if available
 KBUILD_ARFLAGS := $(call ar-option,D)
 
+# enforce correct pointer usage
+KBUILD_CFLAGS	+= $(call cc-option,-Wshadow)
+
 # check for 'asm goto'
 ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC)), y)
 	KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO