[PATCH AUTOSEL 5.17 079/135] s390/preempt: disable __preempt_count_add() optimization for PROFILE_ALL_BRANCHES

Sasha Levin
2022-05-30T13:57:20Z

From: Heiko Carstens <hca@linux.ibm.com>

[ Upstream commit 63678eecec57fc51b778be3da35a397931287170 ]

gcc 12 does not (always) optimize away code that should only be generated
if parameters are constant and within in a certain range. This depends on
various obscure kernel config options, however in particular
PROFILE_ALL_BRANCHES can trigger this compile error:

In function ‘__atomic_add_const’,
inlined from ‘__preempt_count_add.part.0’ at ./arch/s390/include/asm/preempt.h:50:3:
./arch/s390/include/asm/atomic_ops.h:80:9: error: impossible constraint in ‘asm’
80 | asm volatile( \
| ^~~

Workaround this by simply disabling the optimization for
PROFILE_ALL_BRANCHES, since the kernel will be so slow, that this
optimization won't matter at all.

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/s390/include/asm/preempt.h | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index d9d5350cc3ec..bf15da0fedbc 100644
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -46,10 +46,17 @@ static inline bool test_preempt_need_resched(void)

static inline void __preempt_count_add(int val)
{
- if (__builtin_constant_p(val) && (val >= -128) && (val <= 127))
- __atomic_add_const(val, &S390_lowcore.preempt_count);
- else
- __atomic_add(val, &S390_lowcore.preempt_count);
+ /*
+ * With some obscure config options and CONFIG_PROFILE_ALL_BRANCHES
+ * enabled, gcc 12 fails to handle __builtin_constant_p().
+ */
+ if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {
+ if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {
+ __atomic_add_const(val, &S390_lowcore.preempt_count);
+ return;
+ }
+ }
+ __atomic_add(val, &S390_lowcore.preempt_count);
}

static inline void __preempt_count_sub(int val)
--
2.35.1