On Thu, Mar 07, 2019 at 11:39:08AM -0800, Nick Desaulniers wrote: > On Thu, Mar 7, 2019 at 1:15 AM Arnd Bergmann <a...@arndb.de> wrote: > > > > Passing registers containing zero as both the address (NULL pointer) > > and data into cmpxchg_futex_value_locked() leads clang to assign > > the same register for both inputs on ARM, which triggers a warning > > explaining that this instruction has unpredictable behavior on ARMv5. > > > > /tmp/futex-7e740e.s: Assembler messages: > > /tmp/futex-7e740e.s:12713: Warning: source register same as write-back base > > > > This patch was suggested by Mikael Pettersson back in 2011 (!) with gcc-4.4, > > as Mikael wrote: > > "One way of fixing this is to make uaddr an input/output register, since > > "that prevents it from overlapping any other input or output." > > > > but then withdrawn as the warning was determined to be harmless, and it > > apparently never showed up again with later gcc versions. > > > > Now the same problem is back when compiling with clang, and we are trying > > to get clang to build the kernel without warnings, as gcc normally does. > > > > Cc: Mikael Pettersson <mi...@it.uu.se> > > Cc: Mikael Pettersson <mikpeli...@gmail.com> > > Cc: Dave Martin <dave.mar...@arm.com> > > Link: > > https://lore.kernel.org/linux-arm-kernel/20009.45690.158286.161...@pilspetsen.it.uu.se/ > > Signed-off-by: Arnd Bergmann <a...@arndb.de> > > --- > > arch/arm/include/asm/futex.h | 10 +++++----- > > 1 file changed, 5 insertions(+), 5 deletions(-) > > > > diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h > > index 0a46676b4245..79790912974e 100644 > > --- a/arch/arm/include/asm/futex.h > > +++ b/arch/arm/include/asm/futex.h > > @@ -110,13 +110,13 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user > > *uaddr, > > preempt_disable(); > > __ua_flags = uaccess_save_and_enable(); > > __asm__ __volatile__("@futex_atomic_cmpxchg_inatomic\n" > > - "1: " TUSER(ldr) " %1, [%4]\n" > > - " teq %1, %2\n" > > + "1: " TUSER(ldr) " %1, [%2]\n" > > + " teq %1, %3\n" > > " it eq @ explicit IT needed for the 2b label\n" > > - "2: " TUSER(streq) " %3, [%4]\n" > > + "2: " TUSER(streq) " %4, [%2]\n" > > __futex_atomic_ex_table("%5") > > - : "+r" (ret), "=&r" (val) > > - : "r" (oldval), "r" (newval), "r" (uaddr), "Ir" (-EFAULT) > > + : "+&r" (ret), "=&r" (val), "+&r" (uaddr) > > + : "r" (oldval), "r" (newval), "Ir" (-EFAULT) > > : "cc", "memory"); > > uaccess_restore(__ua_flags); > > Underspecification of constraints to extended inline assembly is a > common issue exposed by other compilers (and possibly but in-effect > infrequently compiler upgrades). > So the reordering of the constraints means the in the assembly (notes > for other reviewers): > %2 -> %3 > %3 -> %4 > %4 -> %2 > Yep, looks good to me, thanks for finding this old patch and resending, Arnd!
I don't see what is "underspecified" in the original constraints. Please explain. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
