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

[Peter Zijlstra]

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@xxxxxxxxxxxxxxx>
> > ---
> >  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.