On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote: > On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote: > > > Incidentally, 11276d53 ("locking/static_keys: Add a new static_key > > interface") breaks old-ish compilers (gcc version 4.4.4 20100503 (Red Hat > > 4.4.4-2) (GCC)): > > > > > > > > CC arch/x86/kernel/nmi.o > > In file included from > > /home/build/linux-boris/include/linux/jump_label.h:109, > > from > > /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5, > > from /home/build/linux-boris/include/linux/spinlock.h:88, > > from /home/build/linux-boris/arch/x86/kernel/nmi.c:14: > > /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In function > > ‘nmi_handle’: > > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: warning: asm > > operand 0 probably doesn’t match constraints > > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: error: > > impossible constraint in ‘asm’ > > make[3]: *** [arch/x86/kernel/nmi.o] Error 1 > > make[2]: *** [arch/x86/kernel] Error 2 > > make[1]: *** [arch/x86] Error 2 > > Ugh bugger. > > I bet its that: &((char *)key)[branch] business, an earlier variant > thereof tripped up more recent GCCs too. > > So its an __always_inline function, and both argument are always compile > time constants, @key is the address of an object in static storage (a > global) and @branch is a simple 0/1 at the call site. > > Now we wish to compute (unsigned long)key + branch at compile/link time > to feed to the assembler as an immediate, which should be possible, > given its all 'constants'. > > It just appears GCC is having a hard time with this. > > Let me see if I have a sufficiently old GCC around to play with. Could you feed the below to your compiler? Its a bit cumbersome, but its the next best I could come up with... --- arch/x86/include/asm/jump_label.h | 56 ++++++++++++++++++++++++++++----------- 1 file changed, 41 insertions(+), 15 deletions(-) diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h index 28d7a857f9d1..76c769ae4200 100644 --- a/arch/x86/include/asm/jump_label.h +++ b/arch/x86/include/asm/jump_label.h @@ -16,15 +16,30 @@ # define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC #endif +struct foo { + u8 zero; + u8 one; +}; + static __always_inline bool arch_static_branch(struct static_key *key, bool branch) { - asm_volatile_goto("1:" - ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" - ".pushsection __jump_table, \"aw\" \n\t" - _ASM_ALIGN "\n\t" - _ASM_PTR "1b, %l[l_yes], %c0 \n\t" - ".popsection \n\t" - : : "i" (&((char *)key)[branch]) : : l_yes); + if (!branch) { + asm_volatile_goto("1:" + ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" + ".pushsection __jump_table, \"aw\" \n\t" + _ASM_ALIGN "\n\t" + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" + ".popsection \n\t" + : : "i" (&((struct foo *)key)->zero) : : l_yes); + } else { + asm_volatile_goto("1:" + ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" + ".pushsection __jump_table, \"aw\" \n\t" + _ASM_ALIGN "\n\t" + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" + ".popsection \n\t" + : : "i" (&((struct foo *)key)->one) : : l_yes); + } return false; l_yes: @@ -33,14 +48,25 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch) { - asm_volatile_goto("1:" - ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" - "2:\n\t" - ".pushsection __jump_table, \"aw\" \n\t" - _ASM_ALIGN "\n\t" - _ASM_PTR "1b, %l[l_yes], %c0 \n\t" - ".popsection \n\t" - : : "i" (&((char *)key)[branch]) : : l_yes); + if (!branch) { + asm_volatile_goto("1:" + ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" + "2:\n\t" + ".pushsection __jump_table, \"aw\" \n\t" + _ASM_ALIGN "\n\t" + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" + ".popsection \n\t" + : : "i" (&((struct foo *)key)->zero) : : l_yes); + } else { + asm_volatile_goto("1:" + ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" + "2:\n\t" + ".pushsection __jump_table, \"aw\" \n\t" + _ASM_ALIGN "\n\t" + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" + ".popsection \n\t" + : : "i" (&((struct foo *)key)->one) : : l_yes); + } return false; l_yes: -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html