Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for __downgrade_write()

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for __downgrade_write()
Date	2017-09-07 09:10 +0200
Message-ID	<umZ6O-56x-25@gated-at.bofh.it> (permalink)
References	<umB4t-5b7-3@gated-at.bofh.it> <umB4t-5b7-1@gated-at.bofh.it> <umQdd-7qi-35@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Wed, Sep 06, 2017 at 04:33:02PM -0500, Josh Poimboeuf wrote:
> On Wed, Sep 06, 2017 at 12:26:13AM -0500, Miguel Bernal Marin wrote:
> > kernel/locking/rwsem.o: warning: objtool: downgrade_write()+0x22: call without frame pointer save/setup
> > 
> > The warning means gcc 7.2.0 placed the __downgrade_write() inline asm (and
> > its call instruction) before the frame pointer setup in downgrade_write(),
> > which breaks frame pointer convention and can result in incorrect
> > stack traces.
> > 
> > Force a stack frame to be created before the call instruction by listing
> > the stack pointer as an output operand in the inline asm statement.
> > 
> > Signed-off-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
> > ---
> >  arch/x86/include/asm/rwsem.h | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
> > index d26b6916b935..a749dc6a3103 100644
> > --- a/arch/x86/include/asm/rwsem.h
> > +++ b/arch/x86/include/asm/rwsem.h
> > @@ -205,8 +205,10 @@ static inline void __up_write(struct rw_semaphore *sem)
> >   */
> >  static inline void __downgrade_write(struct rw_semaphore *sem)
> >  {
> > +	register void *__sp asm(_ASM_SP);
> > +
> >  	asm volatile("# beginning __downgrade_write\n\t"
> > -		     LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
> > +		     LOCK_PREFIX _ASM_ADD "%2,(%2)\n\t"
> 
> The '%2' should be changed to '%3'
> 
> 		     LOCK_PREFIX _ASM_ADD "%3,(%2)\n\t"
> 
> because both inputs' indices are shifted by the new output constraint.

Even better would be to used named operands.

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[PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for __downgrade_write() Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com> - 2017-09-06 07:30 +0200
  Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for  __downgrade_write() Josh Poimboeuf <jpoimboe@redhat.com> - 2017-09-06 23:40 +0200
    Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for  __downgrade_write() Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com> - 2017-09-07 00:20 +0200
    Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for  __downgrade_write() Peter Zijlstra <peterz@infradead.org> - 2017-09-07 09:10 +0200

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